ChrisGriffin: What version do you see for both games?
WinterSnowfall: Kingdom Rush is on 4.2.15
Kindgom Rush: Frontiers is on... 1.4.4 (apparently?). It doesn't seem to have ever been updated, at least not recently.
ChrisGriffin: Does one need to be on a linux system to notice that?
WinterSnowfall: You can, at any point in time, check out the Linux installer versions shown in your online game library (on the website).
In that case you should probably contact developer at
https://www.ironhidegames.com/Support and ask them (politely, as I always do) to update linux and perhaps mac versions as well. When I wrote to them I (perhaps wrongly) assumed that they will update other systems as well. Doesn't hurt to ask anyway.
Although maybe it would be better to ask GOG first if they have the files needed to update linux and mac versions. If not, then contact the devs.