Blizzard have dropped a hotfix on classic to fix two issues
The first fix for a bug that was originally sorted in vanilla but existed in classic for the dagger Blade of Eternal Darkness (this fix was intended to be implemented for next patch but they have brought this specific fix forward)
The second fix is for a much more recent issue stemming from the implementation of a 30 instant a day limit. Essentially, if you had filled up your quota for the day, this would also prevent you joining a raid later that day. As of this fix, the raid no longer comes under the same quota which is great news!