I'm not sure if this is truly a "bug" or just a "quirk" of the game, but it seems a bit illogical. When the Dupe uses the hamster wheel manual power generator, the skill assessed (per the jobs screen) is athletics. When they level up on the wheel, the Dupe increases his skill in tinkering.
It seems to me like if the Dupe spends all that time running, then the skill raised should also be running. As it is, the Dupe doesn't get better at power generation even though he spends all that time generating power.
I can understand if this is a balance issue, though, since increasing athletics by using athletics could cause the skill to grow exponentially and create a super-monster power generating Dupe.
Have a dupe run on the hamster wheel generator until he levels up.