DK rune ready but cannot cast spell

In today’s raid I found my DK cannot cast icy touch / blood strike / plague strike even if one of the corresponding rune is available. It only becomes castable when both runes are ready , or another spell consuming a different type of rune being casted. This bug happens fairly frequently, average 1-2 times in any ULD encounter.

Anyone who has experienced the same issue? I knew this bug exists but didn’t expect it happen so frequently, which is super annoying.