You expect read-only replica databases to be in sync with the real production database - especially when AWS is managing them for you cough.
Sometimes this isn’t the case, and there’s a little query that can allow you to diagnose if your replica is having issues.
On the replica instance, run this query for debugging information about the replica’s syncing status:
SHOW SLAVE STATUS;
The result will show an array of statuses, one column in particular is very useful:
Seconds_Behind_Master. This is the measure in seconds of how far behind your replica’s data is from it’s master copy.
For instance, sometimes a reboot is all that’s necessary to fix a stopped replication. If you run
SHOW SLAVE STATUS multiple times you hopefully should see this number decrease.
Hopefully this sheds some light into your issue. Comments, questions, concerns below in the comments section.