012_subtransactions.pl vs clang -fsanitize=undefined

I found that -fsanitize causes the test to fail, going back to REL_10_STABLE,
for any clang in:

1:11.1.0-6
1:12.0.1-19ubuntu3
1:13.0.1-2ubuntu2
1:14.0.0-1ubuntu1

| time ./configure --enable-cassert --enable-debug --enable-tap-tests --with-CC=clang-13 CFLAGS='-fsanitize=undefined'
| time { make -j4 clean; make -j4; } >/dev/null
| time PROVE_TESTS=t/012_subtransactions.pl make -C ./src/test/recovery check
|
| t/012_subtransactions.pl .. 2/?
| # Failed test 'Visible'
| # at t/012_subtransactions.pl line 111.
| # got: '-1'
| # expected: '8128'
| ...
| # Looks like you failed 6 tests of 12.

I haven't found any combination of options which cause it to fail differently,
so I'm not even sure if the problem is in postgres, the test case, clang or
libubsan. Note that optimization seems to avoid the problem, which is why
"kestrel" shows no issue.
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=kestrel&dt=2022-07-23%2022%3A17%3A48

--
Justin