is this a common occurrence?

[Spoekes]

The fight is actually easier if you just ignore the poison and heal through, it doesn't do that much dmg compared to the mp cost of constant esuna. Even easier as sch since you can just /pac embrace and make eos/selene spam healing at 0 mp cost.

Not every poison dot requires cleaning if it makes the healer run out of mp.

This is assuming you're doing it wrong and clean the stacking posion buff at one stack.