Autovacuum of pg_database

Hello again,

we have another strange problem with autovacuum. The process goes well
until it reaches the table pg_database (in some databases, not all),
then it loops over it forever (until I run vacuum full analyze on that
table, simple vacuum doesn't help):

2016-05-05 17:13:33 CEST 34818LOG: automatic vacuum of table
"somedatabase.pg_catalog.pg_database": index scans: 0
pages: 0 removed, 233 remain
tuples: 0 removed, 5817 remain, 0 are dead but not yet removable
buffer usage: 87 hits, 0 misses, 0 dirtied
avg read rate: 0.000 MB/s, avg write rate: 0.000 MB/s
system usage: CPU 0.00s/0.00u sec elapsed 0.00 sec
2016-05-05 17:13:33 CEST 34819LOG: automatic vacuum of table
"somedatabase.pg_catalog.pg_database": index scans: 0
pages: 0 removed, 233 remain
tuples: 0 removed, 5817 remain, 0 are dead but not yet removable
buffer usage: 87 hits, 0 misses, 0 dirtied
avg read rate: 0.000 MB/s, avg write rate: 0.000 MB/s
system usage: CPU 0.00s/0.00u sec elapsed 0.00 sec
2016-05-05 17:13:33 CEST 34820LOG: automatic vacuum of table
"somedatabase.pg_catalog.pg_database": index scans: 0
pages: 0 removed, 233 remain
tuples: 0 removed, 5817 remain, 0 are dead but not yet removable
buffer usage: 87 hits, 0 misses, 0 dirtied
avg read rate: 0.000 MB/s, avg write rate: 0.000 MB/s
system usage: CPU 0.00s/0.00u sec elapsed 0.00 sec
2016-05-05 17:13:34 CEST 34821LOG: automatic vacuum of table
"somedatabase.pg_catalog.pg_database": index scans: 0
pages: 0 removed, 233 remain
tuples: 0 removed, 5817 remain, 0 are dead but not yet removable
buffer usage: 87 hits, 0 misses, 0 dirtied
avg read rate: 0.000 MB/s, avg write rate: 0.000 MB/s
system usage: CPU 0.00s/0.00u sec elapsed 0.00 sec

Not sure if it's a bug or I'm doing something wrong.

Thanks in advance,

Ondřej