Thanks to Jonas Oreland who found this in the code:

>> > @@ -7387,6 +7434,10 @@
>> MYSQL_BIN_LOG::trx_group_commit_leader(group_commit_entry *leader)
>> >
>> > DEBUG_SYNC(leader->thd, "commit_before_get_LOCK_commit_ordered");
>> > mysql_mutex_lock(&LOCK_commit_ordered);
>> > + /**
>> > + * TODO(jonaso): Check with Kristian,
>> > + * if we rotate:d above, this offset is "wrong"
>> > + */
>> > last_commit_pos_offset= commit_offset;
>>
>> Agree, good catch, I think.
>> If we rotate, we already update last_commit_pos_file and _offset. So the code
>> needs to remember if it rotated, and skip the errorneous offset update in this
>> case.

This happens during group commit, if we rotate the binlog as part of the group
commit due to max binlog file size reached.