She can only clear one stack of debuff at a time. If higher priority status such as immobile is present, she'll clear that debuff instead.

No other status was present. Just fire.

Then was this particular ignition a 3 round one.

From my experience a 3 round ignition is treated as 2 ignites. I believe the first removal removes 3 rounds effect the second is the original ignition debuff

This was a ignite from a fire main