After exiling two cards using Opera Love Song, I had two remaining untapped mana and two instants in exile Dreadmaw's Ire and Sazacap's brew, I was able to cawse Sazacaps brew which cost two but unable to cast Dreadmaw's Ire. In the included screenshot note that there is no blue around Dreadmaw's Ire, I tried to cast it anyway to no avail even though I have legal targets. This has happened with other instants as well.

I have experiences this while playing on Mac and my Phone, I only have a screenshot for my Mac.

I expect to be able to cast with valid targets and mana, I am unable.

I have found no workaround though sometimes if you move to a new phase or first cast a different spell you are generally able to then cast the bugged instant. this is of course terrible if order is important for your gameplay which it very much is for this deck.