BUG #16620: Autovacuum does not process certain databases after migration from postgresql 10

The following bug has been logged on the website:

Bug reference: 16620
Logged by: Max Vikharev
Email address: bm(dot)kinder(at)gmail(dot)com
PostgreSQL version: 12.3
Operating system: Linux
Description:

Hi,

There is a cluster with 50 databases. The sizes of tables are very
different, from megabytes to terabytes.

We have been successfully used the aggressive autovacuum for a long time.
After upgrade from 10 to 12.3 we've noticed that autovacuum always process
relations from one database but does not reach others.

I've used
https://www.depesz.com/2020/01/29/which-tables-should-be-auto-vacuumed-or-auto-analyzed/comment-page-1/
to detect existance of relations that should be autovacuumed. There are a
lot of them. At the same time i see that there are free autovacuum workers
that should do this work. But tables are not processed for a long time.

I have logs (log_autovacuum_min_duration = 0) that confirms that only single
database is constantly being autovacuumed. Within pg_stat_user_tables i see
that last time autovacuum / autoanalyze for other databases happened in the
same day when "vacuum to prevent wraparound" worked.

Please help to investigate the problem. Why my relations does not processed
and how can i fix it.

Example table
-[ RECORD 1 ]-------+------------------------------
n_tup_ins | 25599733
n_tup_upd | 2206685374
n_tup_del | 19938580
n_tup_hot_upd | 1863307770
n_live_tup | 186473676
n_dead_tup | 18177988
n_mod_since_analyze | 3250337
last_autovacuum | 2020-08-26 16:43:02.321971+03
last_autoanalyze | 2020-08-26 14:04:19.132566+03
vacuum_count | 1
autovacuum_count | 4078
analyze_count | 8
autoanalyze_count | 357

Autovacuum settings
"autovacuum_max_workers" => 5,
"autovacuum_naptime" => "30s",
"autovacuum_vacuum_threshold" => 5000
"autovacuum_vacuum_scale_factor" => 0.00001
"autovacuum_vacuum_cost_delay" => "10ms"
"autovacuum_vacuum_cost_limit" => 6000,
"autovacuum_analyze_threshold" => 5000,
"autovacuum_analyze_scale_factor" => "0.01"