Electrum told me this input is "frozen". I'm not sure if that's the new default for unconfirmed transactions, but I haven't tried to spend unconfirmed inputs for a while.
That was Electrum's precaution to prevent unconfirmed dust outputs to be spent with your other UTXO in case it has a chain of unconfirmed parents or very low fee.
Here's the relevant commit for reference:
github.com/spesmilo/electrum/commit/27cd0780010462075151efecfacc1f72012efb97