After re-running myself a few times, I managed to hit it again. In tests/openpgp/report.xml, I see:
[...]
<testsuite name="<keyboxd>tests/openpgp/use-exact-key.scm" time="0" package="<keyboxd>tests/openpgp" id="0" timestamp="2022-04-25T16:18:27" hostname="unknown" tests="1" failures="0" errors="0" >
<properties/>
<testcase name="use-exact-key.scm" classname="<keyboxd>tests.openpgp" time="0" >
<failure message="Unknown error." />
</testcase>
<system-out>
Importing public key.
Checking that the most recent, valid signing subkey is used by default
> 8BC90111 3E880CFF F5F77B83 45117079 1EA97479 <
Checking that we can select a specific signing key
> 8BC90111 F5F77B83 1EA97479 <
</system-out>
<system-err>
</system-err>
[...]