MongoDB stale replication, odd timestamp after data flush

Right before my secondary goes stale, it first has this written in the log:

2017-04-25T06:48:02.991+0000 I STORAGE  [DataFileSync] flushing mmaps took 13530ms  for 3150 files

Then it gives me a odd stale message, note the "b" in the oldest available.

2017-04-25T06:50:03.815+0000 I REPL     [ReplicationExecutor] could not find member to sync from
2017-04-25T06:50:03.815+0000 E REPL     [rsBackgroundSync] too stale to catch up -- entering maintenance mode
2017-04-25T06:50:03.815+0000 I REPL     [rsBackgroundSync] our last optime : (term: 6, timestamp: Apr 25 06:32:08:333)
2017-04-25T06:50:03.815+0000 I REPL     [rsBackgroundSync] oldest available is (term: 6, timestamp: Apr 25 06:32:08:3b1)
2017-04-25T06:50:03.815+0000 I REPL     [rsBackgroundSync] See http://dochub.mongodb.org/core/resyncingaverystalereplicasetmember

I am running mongodb 3.2.8. Is that b expected to be in there? Is this expected?