I have successfully completed the Challenge: Rapid Fire 1, a while back. The unlocked weapon, KORD 6P67, is not unlocked for my account, and I'm unable to use it.
This also goes for the Instant Rewards for the Battle Pass. I have purchased the Battle Pass Pro, yet haven't been able to receive the first 6 items included in the Instant Rewards. Perhaps it's glitched thinking that because I didn't buy the Standard Battle Pass, that I haven't unlocked those items? They still have check marks on them as if I had them unlocked.
Images for proof;
