cirrus-ci cross-build interactions?

I had a build on Cirrus CI fail tonight in what I have to assume was
either a problem with caching across builds or some such similar
flakiness. In the Debian task [1] I received this error:

su postgres -c "make -s -j${BUILD_JOBS} world-bin"
In file included from parser.c:25:
./gramparse.h:29:10: fatal error: 'gram.h' file not found
#include "gram.h"
^~~~~~~~
1 error generated.
make[3]: *** [../../../src/Makefile.global:1078: parser.bc] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [common.mk:36: parser-recursive] Error 2
make[1]: *** [Makefile:42: all-backend-recurse] Error 2
make: *** [GNUmakefile:21: world-bin-src-recurse] Error 2

There were no changes in the commits I'd made to either parser.c or
gramparse.h or gram.h. After running "git commit --amend --no-edit"
(with zero changes) to rewrite the commit and forcing pushing the
build [2] seems to be fine. I've double-checked there are no
differences between the commits on the two builds (git diff shows no
output).

Is it possible we're missing some kind of necessary build isolation in
the Cirrus CI scripting?

Thanks,
James Coleman

1: https://cirrus-ci.com/task/6141559258218496
2: https://cirrus-ci.com/build/6309235720978432