Why do I get permafrozen with 100+ ailment duration reduction?

Not sure about other cases, can't remember other ones, but sometimes happens when I frozen plume's area of effect (delirium mod). It doesn't happen always. Maybe 1 in 10 or 1 in 20.

100% reduced ailment duration is not the same as immunity. It gets impacted by mods, whereas 100% avoidance I believe doesn't currently run into those counters. For example, if you are standing in shocked ground with 100% less duration, I believe you will still be shocked because it is constantly being applied. My guess in this case is that some of the mods you are mentioning are probably increasing the duration and counteracting your 100% to under 100%.