PostgreSQL last_vacuum is null for some tables
There could be several reasons for autovacuum to not vacuum the table:

a table storage parameter is configured different from the default: enable_autovacuum = off, autovacuum_vacuum_scale_factor is absurdly high or similar.
Find out by looking at the \d+ output for the table in psql.

the rate of change on the table is so high that autovacuum just cannot keep up.
This is the case if you find autovacuum workers running for the table that never finish. The remedy is to lower autovacuum_vacuum_cost_delay.

data corruption in the table or its indexes make autovacuum fail with an error, so that it never finishes processing the table. Look at the log file to diagnose that.