1.20.1, latest build. If you add any two gems to an affix item that would inflict a negative potion effect on attack, only one potion effect can ever be applied at a time. This is not relevant in the base mod as only one gem can actually inflict a debuff, but it becomes relevant when creating custom gems. Should easily be reproducible with the incandescent inferno gem and adding a gem that debuffs the enemy with something such as weakness.

Note that affixes on the weapons will trigger and stack just fine with gem effects. So a sword with the inferno gem that has a weakness affix will apply flaming detonation and weakness, but creating a custom gem that inflicts, say, sundering and using that with that same sword will not apply all three effects properly.

*This is the same issue I was discussing with you in your Discord. I only encountered it because many of the gems I'm creating apply negative effects