In the bitcoin core doc is explicitly said that conversion from hex to psbt are only allowed for unsigned transactions, or, at most, passing a signed one with the
permitsigdata param set to
true, but again it returns the related unsigned tx.
The converttopsbt RPC converts a network serialized transaction to a PSBT.
This should be used only with createrawtransaction and fundrawtransaction createpsbt and walletcreatefundedpsbt should be used for new applications.
Why signatures aren't allowed/considered while converting?