[2022] Permanent onslaught doesn't count as "buff"

This is correct behaviour. Onslaught itself is never a buff. Onslaught is just a boolean property that your character either has or does not have.

Some things that grant Onslaught for a set duration do so by granting a buff that gives Onslaught - but the buff itself is not onslaught, and that buff can't be scaled by buff effect modifiers because it's only effect is "character has onslaught" which isn't a scalable value.