Bulb can be damaged, even it 'looks' good.
Also on this, multi-meter tells you the reality. Or wrench/screwdriver/battery/ can be used to check the bulb.
Basic principal, to check voltage and bulb first, before checking a fuse...
Tried 3 bulbs, volts and wiring looks good, parking sensor works, reverse bulb doesn’t? Could it be reverse switch then but I didn’t think parking sensors would work if switch is faulty?
Now when you mentioned, it could be damaged ground connection on galvanized metal 'circuit board', which is holding bulbs.
I added additional ground wire to mine right rear turn signal years ago, and turn bulb started to work normally.