Odd, intermittent failure in contrib/pageinspect

whelk failed today [1] with this surprising symptom:

--- snip ---
diff -w -U3 C:/buildfarm/buildenv/HEAD/pgsql.build/contrib/pageinspect/expected/page.out C:/buildfarm/buildenv/HEAD/pgsql.build/contrib/pageinspect/results/page.out
--- C:/buildfarm/buildenv/HEAD/pgsql.build/contrib/pageinspect/expected/page.out 2020-03-08 09:00:35.036254700 +0100
+++ C:/buildfarm/buildenv/HEAD/pgsql.build/contrib/pageinspect/results/page.out 2021-01-18 22:10:10.889655500 +0100
@@ -90,8 +90,8 @@
FROM heap_page_items(get_raw_page('test1', 0)),
LATERAL heap_tuple_infomask_flags(t_infomask, t_infomask2);
t_infomask | t_infomask2 | raw_flags | combined_flags
-------------+-------------+-----------------------------------------------------------+--------------------
- 2816 | 2 | {HEAP_XMIN_COMMITTED,HEAP_XMIN_INVALID,HEAP_XMAX_INVALID} | {HEAP_XMIN_FROZEN}
+------------+-------------+-----------------------------------------+----------------
+ 2304 | 2 | {HEAP_XMIN_COMMITTED,HEAP_XMAX_INVALID} | {}
(1 row)

-- output the decoded flag HEAP_XMIN_FROZEN instead
@@ -99,8 +99,8 @@
FROM heap_page_items(get_raw_page('test1', 0)),
LATERAL heap_tuple_infomask_flags(t_infomask, t_infomask2);
t_infomask | t_infomask2 | raw_flags | combined_flags
-------------+-------------+-----------------------------------------------------------+--------------------
- 2816 | 2 | {HEAP_XMIN_COMMITTED,HEAP_XMIN_INVALID,HEAP_XMAX_INVALID} | {HEAP_XMIN_FROZEN}
+------------+-------------+-----------------------------------------+----------------
+ 2304 | 2 | {HEAP_XMIN_COMMITTED,HEAP_XMAX_INVALID} | {}
(1 row)

-- tests for decoding of combined flags
--- snip ---

Searching the buildfarm logs turned up exactly one previous occurrence,
also on whelk [2]. So I'm not sure what to make of it. Could the
immediately preceding VACUUM FREEZE command have silently skipped this
page for some reason? That'd be a bug I should think.

Also, not really a bug, but why is this test script running exactly
the same query twice in a row? If that's of value, and not just a
copy-and-paste error, the comments sure don't explain why. But what
it looks like is that these queries were different when first added,
and then 58b4cb30a5b made them the same when it probably should have
removed one.

regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=whelk&dt=2021-01-18%2020%3A42%3A13
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=whelk&dt=2020-04-17%2023%3A42%3A10