I just tested this on my PS5 at home and I was able to use relics on the challenger control scheme (L2 + button) while moving. No issues it worked every time.
I was not however, able to use them while moving and basic attacking, so if i was holding down both L2 and R2 then pressed the relic button, that did not go off. Which isnt the issue described here and I believe is intended. But your specific scenario seems to be different.
Now it could be ps4 or xbox only and ill have the QA help me to test those too.
Is there any more information you could give me about what control scheme you use? what casting type? I very much want to be able to find and fix this issue. thanks!