Did you do anything specific for that achievement, or just play in a co-op session and beat the boss? How was your game set up: latest version and DLC installed, old game minus? I'll give it another shot with a friend then.

There's apparently a problem with infected pufferbirds where you need to stun them before slapping, I did it according to this guide and it worked: https://www.trueachievements.com/a291102/i-come-in-peace-achievement
It's also reported here: https://www.playstationtrophies.org/game/journey-to-the-savage-planet/trophy/293698-i-come-in-peace.html
Or maybe you forgot one of the bosses, a baboushka or burglesnatch variant?

I reported about this problem to the Focus Entertainment support center, they haven't answered yet.
No, I don't have Discord.

Bug in achievements in Cave Story game. I started playing yesterday and already got some achievements. However, when I return to the game, the achievements I had already obtained appear blocked in the achievements list.

Ok, I'll try to report the problem on Discord as well, let me know if you hear back!
Very strange, I've never seen anything like this, but you have indeed unlocked some achievements that are then blocked in the main screen. Try waiting a few days or unlocking a few more, maybe it will work itself out.

I'm also having a bug with the achievements in CaveStory. I got three weapons and defeated two bosses, then I exited the game.

Today when I started the game again, I had no achievements. Even those for getting the first weapon, the map or defeating the bosses.

Edit: It seems that when I continue the game, all previous achievements are deleted and only the new achievements appear in the list. I have 11 achievements, but as I've played in three sessions, only the last two achievements appear.

Very strange, I strongly suggest to open a ticket with GOG support since the bug seems to be affecting the profiles.

https://support.gog.com

If you find the bot, write your report and when the bot gives you default answers tell it that it wasn't helpful, at some point it will ask for your email and then you can rewrite the report and a ticket will be created to which a human will answer after a few weeks/months.

I can also confirm "I Come in Peace" works, I just unlocked it as well, using the solution listed here.

In addition, Creaks seems to have functioning Galaxy achievements now.

I got the following response regarding the lack of achievements in Escape From Naraka:

Thank you for your email, also thank you for your suggestion and information! I will tell it to the development team and our publisher!

I also asked about leaderboards, since that's missing in the Game Features box, but mentioned in the description. It looks like another case of a game released without leaderboards, because they were tied to Steam, but the description gets copied and pasted unmodified. If that doesn't get sorted early next year, I'll try to remember to open a support ticket.

Is there any way to add a missing "100% achievements" tile on the profile page?

Unto the End had a test achievement that wasn't supposed to be there. The developer removed it, so my completion jumped to 100%. No way to unlock an achievement that brings the total to 100% means no 100% tile on the profile.

I was previously told support "don't have the tools" to manually add those to a profile, so that's not an option. GOG Galaxy is currently still listing the removed achievement and <100% completion.

I missed the fact that Unto the End also had a non working test achievement.
Unfortunately to my knowledge there is no way to do what you say, if not reset all the achievements and start over, but GOG support doesn't offer this possibility anymore since their customer support is deteriorated and continuously lagging behind.

It was the "Secret achievement" in my previous post. Some achievements inexplicably wouldn't unlock, so after exhausting my usual options, I contacted the developer again. We didn't figure out why those were finicky, but I did unlock them somehow. At the same time, he said there's no secret achievement, and here we are.
You mean support could erase just my own achievement progress for the game? If it comes down to it, I'd be okay with that.
I've heard that support manually unlocked a bugged achievement for someone once. Is that true? If so, maybe they could lock one of mine, and then I'd unlock it again for the 100%.

Happy New Year!

Yes, the support used to be able to delete all unlocked achievements of a game (I don't know if they could do it for a single achievement), but now I've heard that unfortunately they don't accept these requests anymore, maybe because they want to streamline their incoming tickets. Trying to ask doesn't cost anything though.

And yes, support used to unlock broken achievements for those who requested it and provide proof of having the requirements, though I haven't tried it yet. Also in this case I don't know if they still do it.

Happy New Year to you too!

Cave Story + received an update, and APPARENTLY the achievements have been fixed.

As I hadn't played the GOG version yet, I took the test, unlocked two achievements, exited the game, came back and they're still there, so it looks like the problem has been fixed.

Yes, this issue seems to be fixed, but some trophies still appear to be bugged. I (and many others) didn't unlock an early one, defeating Balrog for the first time.

The House of Da Vinci games appear to have 'hidden' Galaxy achievements, possibly brought in a recent update.

In the meantime they have become visible along with The House of Da Vinci 2 achievements and they seem to work, so I'm removing them from the list now!