I have already said that it might have been really bad luck, but I find it weird that an enemy which isn't even listed as uncommon is that rare.And no its not a bug Imps dont have a 100% chance to spawn. So from time to time you may not see one for quite long similar situation like with sharks or chaos elementals.
What was your fishing spot? The location of the fishing spot can easily affect which mobs spawn in a certain rarity.After fishing in lava for about 3 hours I didn't even get one. And I was in the underworld letting my vampire frogs kill every imp that spawned. Imps really are not common. I didn't even get the banner for them in that time.