Sean sent this update earlier (before Jon noticed the issue) and agreed to let me forward it here.
> db1047 has been upgraded to MariaDB 10, however it is not yet replicating
> from m2 because I ran into a replication bug on db1046, the m2 slave I had
> intended to make db1047's master for the log db. Until I get that glitch
> sorted I have db1047 federating the log tables instead using the new
> CONNECT engine, which seems fine so far.
The data on db1048 looks healthy (and is continuing to accumulate incoming events), which supports Dan's hunch that the issue lies with replication. So there is still nothing that suggests data loss. But once the migration is over, we should cross-reference the database with the log files and confirm this definitively.