I don't know if its illegal or not, but the "Holmes and stolen stones" Yggdrasil game is definitely a major user of "change game behavior when close to jackpot". The sheer number of times when I have found myself in a free spins round waiting in vain for the 5th gem with 4-5 free spins left is colossal, well over 500. It's statistically impossible that you get to 4 gems almost every round in a thousand rounds, but not even once to 5 gems. It's totally out of proportion.
This would be understandable if observable Gem frequency per reel was similar to Free Spin or Bonus icons, which are much more rare and you'll practically never see 4 (or even 3) of them land on the same reel in 10 spins (and having 3 or 4 of them on screen simultaneously is extremely rare, unlike with gems which experience this frequently), but this has to be a case of taunting players with fake "near misses" that are vastly out of proportion to what you can actually see on the reel and what you are told in the rules.
If you can easily verify that each reel has a 10 to 30% of producing a gem on each spin, then it's simply out of bounds of any imaginable variance that it fails to ever produce a fifth gem on many thousands of spins that it performs in this situation. The chance of that happening is less than one in a billion. And it consistently happens at all bet sizes, from €0.20 to €4.
I will play a bit more and write down these occurrences in detail so someone can send a proper report to UKGC.