Bit the bullet, bisected v5.5..v5.6 and found this:
0ddad21d3e99c743a3aa473121dc5561679e26bb is the first bad commit
commit 0ddad21d3e99c743a3aa473121dc5561679e26bb
Author: Linus Torvalds <[email protected]>
Date: Mon Dec 9 09:48:27 2019 -0800
pipe: use exclusive waits when reading or writing
[...]
[ Note! This kernel improvement seems to be very good at triggering a
race condition in the make jobserver (in GNU make 4.2.1) for me. It's a
long known bug that was fixed back in June 2017 by GNU make commit
b552b0525198 ("[SV 51159] Use a non-blocking read with pselect to
avoid hangs.").
But there wasn't a new release of GNU make until 4.3 on Jan 19 2020,
so a number of distributions may still have the buggy version. Some
have backported the fix to their 4.2.1 release, though, and even
without the fix it's quite timing-dependent whether the bug actually
is hit. ]
[...]
After updating "make" to 4.3 all is well.
Feynman was right again. ;)
--
The truth always turns out to be simpler than you thought.
Richard Feynman