Maybe I'm wrong, but it looks like that the handling of the height/hash union for getblockstats first argument seems inconsistent or wrong.

bitcoin-cli getblockstats 10000 works
bitcoin-cli getblockstats '"<hash>"' works
bitcoin-cli getblockstats <hash> gives "Error parsing JSON"

Ignoring that a '"<hash>"' (simple string) is probably not a valid json text, it seems to be inconsistent with other calls like getblock <hash>