Multisig encoding is only needed for long broadcast texts and long asset descriptions (at least these are the only two cases I can think of).
I suggest limit the length of broadcast text and description in the GUI so that these always can be encoded in op_return. Fixing the underlying bug may require more work than it's worth, since this is a rare issue.
If CW tries to encode the transaction with multisig, it usually fails silently.
It works only if there are two UTXOs available per multisig, like here: https://testnet.xchain.io/tx/89cdeea3c0bd55e6225975aa80061b711f131cbfdf3391121dd6ed6b89a7c7e5 https://www.blocktrail.com/tBTC/tx/89cdeea3c0bd55e6225975aa80061b711f131cbfdf3391121dd6ed6b89a7c7e5
Multisig encoding is only needed for long broadcast texts and long asset descriptions (at least these are the only two cases I can think of).
I suggest limit the length of broadcast text and description in the GUI so that these always can be encoded in op_return. Fixing the underlying bug may require more work than it's worth, since this is a rare issue.