Errors with `walletcreatefundedpsbt` & non-solvable UTXOs
Yes. Bitcoin Core will not be able to select for coins that are not solvable. This is because, for fee estimation, it must know how large the final transaction will be, but it cannot do this without knowing everything needed to construct the final transaction. 

The outputs don't need to be spendable (i.e. you don't need to have the private keys for them) because Core knows how large a signature will be so it creates a dummy signature. But it needs to have everything else such as scripts and public keys. Without those, the output is not solvable and cannot be used in coin selection.