BUG #16098: unexplained autovacuum to prevent wraparound

The following bug has been logged on the website:

Bug reference:      16098
Logged by:          Alessandro Ferraresi
Email address:      alessandro.ferraresi1@gmail.com
PostgreSQL version: 10.6
Operating system:   RHEL 7
Description:

Hello there,

I'm experiencing a weird behavior on 10.6 where I see autovacuum running to
prevent wraparound even if autovacuum_freeze_max_age is far away.
The database is brand-new and some tables have been loaded (using copy
command), below some details/parameters:  

pgxxxx=> show autovacuum_freeze_max_age;
 autovacuum_freeze_max_age
---------------------------
 200000000

 oldest_current_xid | percent_towards_wraparound |
percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
               3210 |                          0 |
      0

The tables involved have these custom parameters:

Options: 
fillfactor=80, 
autovacuum_enabled=true, 
autovacuum_vacuum_scale_factor=0.00, 
autovacuum_analyze_scale_factor=0.00, 
autovacuum_vacuum_threshold=500000, 
autovacuum_analyze_threshold=50000

From pg stat activity:

       age               | state  |   backend_type
                        query

-----------------------+--------+---------------------------+---------------------------------------------------------------------------------
00:18:32.899444 | active | autovacuum worker | autovacuum: VACUUM
haas.table1 (to prevent wraparound)
00:26:57.660416 | active | autovacuum worker | autovacuum: VACUUM
haas.table2 (to prevent wraparound)
00:26:54.848344 | active | autovacuum worker | autovacuum: VACUUM
haas.table3 (to prevent wraparound)

What could be the root cause of this? I believe this is  a bug as I can't
find any other explanation.

Thanks
Alessandro