They are not, unfortunately (fortunately?). Just a standard table.
If it helps, the autovacuum always seems to be in the phase "cleaning up indexes" when I look at pg_stat_progress_vacuum.
We recently upgraded about 1500 Postgres clusters from 9.3 to 10.1 and we are seeing similar issues. Specifically:
* An autovacuum worker stuck vacuuming regular tables for days until they are killed (I've seen "vacuuming indexes" phase), and they are showing 99% CPU in top. pg_cancel|terminate_backend does not usually kill the process, only a hard kill does. In the latter case, two (out of four) times it also crashed Postgres.
* There have been about 10 examples of btree index corruption, across a handful of databases, in different data centres. In two of the cases, it's possible there were hardware issues as the hypervisor those two VM's were running on crashed.