If i stress test, and it fails either IBT or prime, i would consider it unstable.
The problem is most people dont know how to run IBT properly, so u may pass a test with IBT but fail with a Prime blend test.
Running IBT just on standard is wrong, as ideally u need to run IBT with no running background processes and a custom amount of ram, so u get at least 75% ram usage during the test.
Prime blend test does this, so that is why Prime can find an unstable overclock, but it usually takes a lot longer.
bump the vcore another notch, and test again