Engine heat will cause most of the excess oil from the air cleaner to deposit in the bottom of the airbox. There's a drain for that under the airbox. Looks like a short piece of plastic tubing, flattened on one end.
It's "normal" but most of it can be avoided by not over-oiling the air cleaner after you clean it.
Within reason it does no harm and has a minor side benefit in that it traps dust particles that might get past the main filter.
If a float sticks the airbox and crankcase can fill with gas/oil mixture, which is a whole other ballgame. Makes sure it's just oil.