I have also considered that it might be a display bug with the recently converted monster's health bar, but that isn't the case. Once I convert a monster with 1 hit, doing about 5% of its full health in damage, I can then wait for the conversion to wear off and then kill it with 1 punch of my fists!
I seem to remember this being a very old bug for the paladin (back in D2C at least....) It caused a problem because at that time many paladins used conversion and thorns - this was before the 50% physical resistance implemented in Hell D2X - and conversion became much more powerful than it should have been. However, I was under the impression that this was fixed in an earlier patch... maybe Blizzard made their previous bug recur accidentally?
After some prolonged testing, it seems that the bug kicks in 90% of the time. I've tested it on D2C on the realms and on LoD single player. Both were version 1.10. Prehaps someone could test it on the LoD realms? Though I don't see why it should be different. In any case, this bug should be added to the list of 1.10 bugs.