That is indeed a puzzle... I've honestly never seen anything like this. I would hazard a guess that it was a bug... as the UTXO generated is still unspent and given the current price of BTC, I would expect it would have been moved/sold by now if it was 'stolen'.

Quite how you can "accidentally" sign a second transaction is a complete mystery and it seems it would be incredibly difficult to replicate the issue. I've certainly not experienced anything similar.

If Ledger support are unable to assist, I doubt anyone here will be. Have you tried raising an issue on the Electrum github? https://github.com/spesmilo/electrum/issues