I found they released their "new" game "Kingdom Rush Vengeance" which was released 14 months ago on Steam here yesterday. So I think it's necessary to talk about this studio.

1. I bought one of their games "Kingdom Rush Frontiers" on GOG years ago.

2. They released one update on January 9, 2020 on Steam.
https://steamcommunity.com/app/458710/discussions/0/1743389952524783916/

3. I found the GOG version was still outdated on November 2020. I contacted GOG support and asked the devs on Steam forum myself.
https://www.gog.com/forum/general/games_that_treat_gog_customers_as_second_class_citizens_v2/post2015

4. More than one month later, they finally updated the GOG version.

5. This year, they released another big update on September 1 on Steam.
https://steamcommunity.com/app/458710/eventcomments/3039354735042916564?snr=1_2108_9__2107

6. Again, until now, GOG version is outdated.
And this time they even successfully released another game here yesterday.
https://www.gog.com/forum/general/release_kingdom_rush_vengeance_9f77c

How could this happen? How can the devs and even GOG treat their customers like this?

Well, I can't stand unwinnable endless-type levels (even though I like "losing is fun" games like Space Station 13, go figure) and microtransactions, so that missing update would get a "good riddance!" from me. Not a good thing that a game's missing features, but I can't bring myself to care about that particular update.
At least Vengeance is here, more than three years since it was released elsewhere.
And it just got a Chinese-themed update apparently, wonder if it's included or it'd take another three years to see it here. Does anyone know?

Most games here are like that unfortunately, and then they get front stage status and bumped into bestselling, I guess most people don't care about these things as whenever I bring it up it brings out the downvotes.

I like giving my money to hard working devs, it's hard to find them but they exist.