Somehow the lxd containers being used for autopkgtest are, likely, different. x64 seems to be running privileged containers for need_root tests, while armhf is not (orelse x64 selfpkgtests wouldn't pass either, like demonstrated in previous comment).
I'll suggest a hints-ubuntu test marking this as bad-test, but it seems that the environment is bad, and not the test.
Somehow the lxd containers being used for autopkgtest are, likely, different. x64 seems to be running privileged containers for need_root tests, while armhf is not (orelse x64 selfpkgtests wouldn't pass either, like demonstrated in previous comment).
I'll suggest a hints-ubuntu test marking this as bad-test, but it seems that the environment is bad, and not the test.