I have Mount and Blade 2 Banner Lord in my library. Bought it when it was first available to purchase here. However, when I see it on the store page it is not marked as "In Library". All of my other games are marked correctly. However this one isn't.

It's as if I don't own it, yet I do? Has anyone ever experienced this with this game or any game for that matter?

Wondering if it's just a glitch or bug with the site?

What you have in your library has a different internal id than the game in the store despite being the same game. I Don't remember what causes this issue.

I had this happen after I claimed VirtuaVerse during the giveaway, but I cannot tell you what fixed it as the website did it on its own. I'd contact support in your case.

That could be caused by you having a different edition of the same game in your library as compared versus the version that is listed on the store page that you are looking at (i.e. if I own a Deluxe Edition of a game on GOG, then it will still have the Standard Edition of the same game marked as unowned on the store page, even though I actually already do own the Standard edition as part of my Deluxe Edition that is in my library).

But if the game is actually in your library, as your OP seems to state that it is, then I'm not sure why this issue would be a big deal.

It appears the store sells a package containing the game while you simply have the game. Which means you got the game through some sort of promotion (or maybe gifted?). this doesn't go away as far as I know.

For whatever reason, there are sometimes packages (bundles) seemingly comprised solely of the game, and then there's the game itself. Since these have different product IDs, when you somehow end up owning only the game, the package isn't marked as owned.
https://www.gogdb.org/products?search=bannerlord

I'm in this boat with Oblivion. I got only the game itself through Amazon Prime. The store wraps up the game in a package, so that isn't marked as owned.

The store is also complete garbage at correctly displaying ownership for packages and DLC. You're very lucky if you haven't run into that issue, given the size of your library. Just a few examples:
https://www.gog.com/forum/general/please_show_owned_games_as_owned/post5

Hmmm you know, come to think of it, there was a promotion that had Banner Lord as a package deal on GoG. I got it then. That could be what the issue is that you guys have stated. Thanks for the assistance. Also, yeah I have 4 kids under 4, and my library is only getting bigger which means so is my backlog.

But All the games I have through, Steam, Epic, and GoG will basically be given to them if they want to play them. When they're older of course.

looks like you have to rebuy it