Monitoring for and dealing with sudden jump in TOAST bloat
If you want to check how bloated the table is, you should install the pgstattuple extension and execute pgstattuple() on the suspected table. If the table is large, as in your case, you can use pgstattuple_approx() to get an approximation.
Regarding your hypothesis, I don't think that Multi-AZ can explain your issue, as PostgreSQL Multi-AZ replication is synchronous. Network issues between master and standby would halt the activity on the master.
Other possible explanation might be:

Autovacuum not being able to catch up with many deletes or updates.
This can happen because of database load that is unrelated to that
query. Look at your logs and CloudWatch metrics for clues
If you have a regular read-replica (not Multi-AZ) then your hypothesis could explain what has happened. There is a CloudWatch metric that shows the replication lag.