[2020] Ignites stop doing damage below 0.5 sec duration

This is technically correct, beacause ailments are ignored if their duration would be below 300ms. This is primarily to avoid the performance cost of adding and removing lots of debuffs too quickly, in cases where they have little time to take effect. This is less true for damaging ailments than non-damaging ones due to their effect being effectively cumulative and the damage they cause resulting in ongoing changes (loss of life) that persist beyond the ailments duration, but very short damaging ailments also have their own issues with accuracy of duration - any ignite that this is skipping would have ~10% more/less effective duration based on when in a frame it was added, and that error gets larger as the duration gets shorter.

I don't think this has actually been possible in-game before now for ignite, so it's never come up. This is complicated slightly more by the fact that the other damaging ailments, bleeding and poison, don't currently have that restriction - simply because they weren't ailments for most of their time in the game, and when they were changed to be ailments, they didn't get that restriction at the time.

There is consideration happening currently about removing or lowering this duration threshold on ignite, and the potential consequences, so I can't say for sure what will be changed here yet.

It works fine with one ring (EDIT: My testing of this was flawed, there are cases where this cutoff threshold threshold can be hit with just 1 ring) - the cutoff is 300ms, not 500. Two rings is 180% reduced duration, which is so far past the point of not having a duration at all we didn't expect anyone to find enough duration bonuses to counter that and try to ignite in the first place, and even then there's only a small band of durations which get caught by this case.

Yeah, my quick test of this was flawed due to a small amount of increased ailment duraiton on my character that I didn't catch, it's definitely possible to hit this threshold with just one ring if you have no investment into increasing the duration.