This parameter prevents Patroni from switching off the synchronous replication on the primary when no synchronous standby candidates are available. As a downside, the primary is not be available for writes (unless the Postgres transaction explicitly turns of synchronous_mode), blocking all client write requests until at least one synchronous replica comes up.

https://patroni.readthedocs.io/en/latest/replication_modes.h...

edit: seems I missed this discussion on twitter: https://twitter.com/jepsen_io/status/1265626035380346881

Can be reproduced even on a single node postgres. Just hammer it with inserts and maintain a local counter for inserts performed. Then, kill9 the postgres process. You'd expect your local counter to match the actual rows inserted, but you'll find that your counter will always be "less" than the actual rows inserted. Like any "networked" system, it is possible to lose commit acknowledgments even if the commit itself was successful.

So yes, you've not "lost" transactions per se. You've "gained" them, but it is still a data issue in either case.