If you export your single sig xpub and whack that in a multisig, it will have the wrong derivation path, meaning you won't be able to sign psbts (the workaround is to change the config file to show the correct derivation path)

Also, your single sig xpub with single sig derivation path may leak your single sig balance / transactions

YMMV depending on the wallet you use, and whether you use a hardware or software signer

The point is, that each derivation path results in a new xpub, and some multisig wallets / signers are more forgiving than others in dealing with this