So what I think now is, did Mycelium get stuck in choosing which input address (bc1q1xxx/bc1q2xxx) to sign the transaction and displayed an error? Clearly both address had the required funds to move.
Impossible to know without seeing the logs from the application. The "Transaction signing failed" error is a relatively generic error that doesn't really tell you much other than the app was unable to sign your transaction
Why it was able to sign the transaction when you spent BOTH UTXOs in one transaction, but could not do so when you were just spending ONE UTXO... I guess we'll never know.
Given some of the "syncing" issues that Mycelium users have had since the latest update... it's possible the app was simply unable to confirm the UTXOs were "valid" so the creation/signing of the transaction simply failed... and then by the time you decided to try sending everything, it was able to sync OK and confirm the UTXOs were valid, so the process worked fine. It's also possible that Mycelium doesn't work like this at all and will happily sign transactions containing invalid UTXOs
I'm not very familiar with the underlying codebase to be honest.
I imagine attempting to troubleshoot this would be very difficult as it will not doubt prove to be hard to replicate the issue!