It is normal, in the sense it happened in live and was a known bug. It's not limited to just mobs, players can do it to other players and sometimes to mobs as well, just like your horse can attack while you're mounted on it sometimes.
Basically, if a melee enemy is going to attack you with basic attack, even archers sometimes, you never want to try to trade blows with basic melee. A pvp example of this working: player X and player Y. X and Y get in a fight, X targets Y with a basic attack, but then stops in place, leaving his lock-on for Y still active, but does not load a skill. Now if player Y tries to attack X within a short time of this action, there is a chance X will auto attack as soon as Y is in range, even though X has not issued another command, and there will also be no animation. It's essentially a free hit, and is done defensively, by choosing a target and letting them come to you. It's not a guaranteed event though, it don't always work. But that's the conditions for activating this bug, lock and don't load a skill, but let the target try to melee you. Archers will occasionally pull off the same with their bow out, lock and stand there, then counter with a kick while holding your bow out, even though the bow has no melee.
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Discord: Tonkatunk#7079
https://github.com/Tonkatunk/Mabi-Pro-Lazyfae-Mods