Possibly an issue, but only if there were only one enemy and you could hit them reliably and they didn't have a ranged attack and their top priority was to get close to/move past your character (instead of another). It seems like a specific enough situation that a little cheese probably isn't a big deal.
This isn't really all that specific and the cheese isn't really that little. Assuming your shove didn't cause a fall that outright killed the enemy (which is not acceptable cheese in of itself - especially since your halfling can push a 300 kilo ogre, or an 8 legged super giant spider, like a feather/Bambi on ice, but not the other way around), you would get an attack in exchange for a bonus action - unless the enemy change targets. Which it shouldn't as it would require omniscient meta-knowledge. And even so there would need to be an alternative melee target reachable in melee without triggering opportunity attacks in the process. Or the enemy would need a viable ranged alternative (forcing a melee monster to go with a clearly inferior ranged option pretty much wins any fight). Dueling the deadliest swordsman in all of Faerun? The player wins unless the swordsman also happens to be Robin Hood.
Larian would need a much more advanced AI capable of learning tactics to adapt to the player (or cheat with omniscience). And this is just one of likely hundreds of things negatively affected by the cheese.
