I've seen bugs like this but never in a reproducible way. Retargeting the mob always seems to help. This can also happen with pet hp bars, and can be very frustrating if you think your pet is fine, only to find out he is about to die. Sometimes you might also start the fight out on a strange footing when the mob has a rune-type spell buff on before you attack. Sometimes I'll open with a root that does damage but only do 1% or less to the mob, even though the game tells me I did 190 dmg, but really the damage absorber is sucking some of it up. After a few hundred dmg this goes away, and i don't think it should be much of an issue at your level, but it seemed worth mentioning.
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