I have recently replaced a 9 speed chain with a 10 speed one on a 9 speed cassette. I read this is compatible. Shifting is fine and quieter but it is now jumping on the smaller cogs 6-9. Is this a common occurrence? Never had any skipping on 9 speed chain though it was a lot noisier.
This feels like a new chain on a worn cassette problem. The fix is to replace the cassette, and soon before your chain suffers accelerated wear.
Assuming you have 9 speed shifter and rear mech still, the slightly narrower chain won't really make any difference while pedalling, and shifts might be slightly sloppier.
I run a 10 speed chain on a 9 speed cassette without issue, because its what I had at the time.
After trying many things I think I've found what the underlying issue is.Its not the chain because was even still doing it on a different 9 speed chain.The issue i think lies with a gear shifter by sunrace,an indexed manual thumb shifter.I dont know the reason why but it slips into other gears when peddling hard.its not the cassette as nothing is skipping over the teeth.