--source include/restart_mysqld.inc results in a test failure in Spider suites, with no output from mysqltest. Putting the following MTR test case to storage/spider/mysql-test/spider/bugfix/t reproduces the bug.
This doesn't happen at least in the main suite and the include file is used elsewhere. So, I expect that the problem is Spider-specific.
The Spider hangs on the server startup on 10.4+ (
MDEV-22979) and thus the failure is expected on 10.4+. However, even on 10.2, the test fails. This is possibly due to a different reason.