Here is the scenario
Lets say the current leader A is at zxid 80.
A participant B with zxid 81 joins and gets a message from leader TRUNC,80
B then calculates the length of log up till zxid 80. The actual length is, say 450, but because of the bug, the value it calculates is 420. B truncates the log to size 420.
When loadDatabase is called again, the log is replayed till 79 because log record 80 isn't complete.
The node B doesn't have the change that had zxid 80. The leader will not send change 80 to B either.
In my manual repro, the change with zxid 80 was a create. I could see the created node when I connected to A but not when connected to B.