Chain skipping mid cassette after a clean
It's possible that during cleaning a grain of sand, for example, has become stuck in the chain causing a stiff link which does not travel cleanly over the cogs. Cleaning chains and cogs often involves a significant amount of moving dirt around before extracting it.

A simple test for this is to move the chain through both hands bending every link. You'll find a stiff link very easily.

If that's the problem, clean the link until the stiffness goes, re-lube and you should have no further problems.