Investigate failing assertion in TxSubmission.Inbound

The following assertion is failed in the nightly consensus tests:
https://github.com/input-output-hk/ouroboros-network/blob/6a2b8922856460cccdc7015d33a14ebd28696d14/ouroboros-network/src/Ouroboros/Network/TxSubmission/Inbound.hs#L201-L204
For more details, see https://input-output-rnd.slack.com/archives/GCW2S19CM/p1601951773003900 and in particular https://input-output-rnd.slack.com/archives/GCW2S19CM/p1602269445020300?thread_ts=1601951773.003900&cid=GCW2S19CM

Bisecting has shown that the failure was introduced in https://github.com/input-output-hk/ouroboros-network/pull/2644/, which updates the ledger dependency. Among the changes to the ledger was an update to the transaction generator that we use in the failing consensus test. I don't think there is a bug in that generator or that the ledger is to blame, but that the change in behaviour of the generator is just triggering the bug that was there all along.

To reproduce, see https://github.com/input-output-hk/ouroboros-network/tree/nfrisby/debug-nightly