postgres 9.4.5 - pg_serial not decreasing

Hello,

Few weeks ago our customer experienced some spikes on his Postgres 9.4.5
database. We've been alerted of a significant disk usage increase and
pg_serial seemed to be the culprit.

We noticed a lot of those errors in the logs

" could not truncate directory "pg_serial": apparent wraparound "

No more errors now but the pg_serial is still 8G big and doesn't seem to
decrease.
The only thing I could find related to this was an old bug from 2011 which
might be fixed now.

We wonder what is responsible for cleaning up the old transactions in
pg_serial and how it could dysfunction in this case.

Would you have any clue ?

Cheers,