Why is VACUUM FULL necessary here?
Your table clearly has had a troubled history and became extremely bloated.  However, nothing in the log files you showed us indicate what the nature of that history is.

It could be that autovacuum was turned off for a time, or misconfigured and so ran too slow to keep up, or that there was a transaction held open for a very long time which prevented vacuum from being effective, or maybe someone was constantly locking the table in a mode stronger than ROW EXCLUSIVE, causing autovacuum to yield before finishing everytime it tried to run.