As the title says. It used to work, but now it doesn't. All games are affected. I don't think I have changed anything... probably a bug that came with an update?

I never understand this. Why does it matter how long you've been playing a game for?

If you enjoy the game, fine, keep playing.
If you don't enjoy it, stop playing.

Either way, it shouldn't matter how long you were playing it.

I don't know about everybody else, but personally, I have so many memories from my childhood and I can only use vague estimates(countless hours, just one more turn and then it was morning, months on end) to express how much I played certain games.

Also, maybe a counter would help me drop my addiction with Hearthstone, but Blizzard, I think intentionally, didn't implement one. I have just crossed 10.000 wins in that game. Considering approximately 50% win rate, that means about 20.000 games played. When you think that a game lasts 7-15 minutes, the extent of my addiction becomes clear.

And now that I think about it, the wins counter was introduced after the game came out of beta, so I must have played more than that.

Well, if you were in the U.S. I'd say you could get the information by filing a FOIA request with the NSA.

But you're probably just out of luck.

As I have noticed, you need to be online and connected in GOG Galaxy while playing the game for game time tracking to work.
This leads to erroneous time tracking.

Hasn't worked for quite some time. There's a thread about this every so often.

I believe you want to be posting about your issue here in the sticky:

https://www.gog.com/forum/general/gog_galaxy_official_discussion_thread

That's the thread the developers track and will bring it to their attention once again.

Hope this helps.