Why is my table continuousely written? -> XID issue?

I had a look into what actually changed in the table. At the first
write that I grabbed, four rows in that segment had such a change:

117ee000 77 00 00 00 00 df b8 82 8e a4 00 00 64 00 a0 00 |w...........d...|
117ee000 77 00 00 00 f0 22 b4 f3 68 d3 00 00 64 00 a0 00 |w...."..h...d...|

117ee0a0 f8 5c 00 00 f8 5c 00 00 09 00 00 00 06 00 f7 8b |.\...\..........|
117ee0b0 13 00 04 00 93 00 18 0d d8 27 fe 01 00 00 00 00 |.........'......|
117ee0b0 13 00 04 00 93 01 18 0d d8 27 fe 01 00 00 00 00 |.........'......|

1aad8000 77 00 00 00 30 fc 8c a5 80 8a 00 00 58 00 a8 00 |w...0.......X...|
1aad8000 77 00 00 00 e8 12 8a f3 6e 5b 00 00 58 00 a8 00 |w.......n[..X...|

1aad80a0 f6 5d 00 00 00 00 00 00 |.........]......|
1aad80b0 09 00 00 00 06 00 6c d5 10 00 04 00 03 08 18 0d |......l.........|
1aad80b0 09 00 00 00 06 00 6c d5 10 00 04 00 03 09 18 0d |......l.........|

If I get that right, then it is the HEAP_XMIN_COMMITTED from
t_infomask that got set for the rows.

The next write is a bit different:

0000a000 76 00 00 00 60 9d 84 d0 23 4c 04 00 70 00 50 01 |v...`...#L..p.P.|
0000a000 78 00 00 00 80 81 e9 05 78 0d 04 00 70 00 50 01 |x.......x...p.P.|

0000a690 cf 51 00 00 00 00 00 00 |504c54...Q......|
0000a690 cf 51 00 00 13 5e 00 00 |504c54...Q...^..|
0000a6a0 00 00 00 00 06 00 05 00 12 00 04 00 03 2b 18 0d |.............+..|
0000a6a0 00 00 00 00 06 00 05 00 12 00 04 00 93 23 18 0d |.............#..|

003ec000 76 00 00 00 30 98 85 d0 59 e9 04 00 60 00 48 01 |v...0...Y...`.H.|
003ec000 78 00 00 00 08 33 1b 03 bd 97 04 00 60 00 48 01 |x....3......`.H.|

003eddf0 cf 51 00 00 00 00 00 00 00 00 00 00 06 00 f6 01 |.Q..............|
003eddf0 cf 51 00 00 13 5e 00 00 00 00 00 00 06 00 f6 01 |.Q...^..........|
003ede00 02 00 04 00 03 2b 18 0d f3 56 2a 00 00 00 00 00 |.....+...V*.....|
003ede00 02 00 04 00 93 23 18 0d f3 56 2a 00 00 00 00 00 |.....#...V*.....|

It seems here is the t_xmax for the row updated, and also flags in
t_infomask changed - and there are about 1000 rows (of some 3 mio.
that are contained in this segment) where this is done.

So if I get this right, it looks like some out-of-band housekeeping is
done here.
Now, while I would understand that this is done as occasion offers
when a record is read anyway, and I do not think that I grok the
XID housekeeping in fullness, I also do not think that I have any open
transactions that would persist a server restart. So I was under the
impression that a server restart plus a vacuum FULL, without any
further update/delete operations, should resolve all the needs for
such housekeeping for the time being, and quiesce the situation.

Apparently it does not. So, then, what does?