similar issue for me, had a flash creature and mana for it but didn't give me a chance to cast it, after attackers

also happened with accumulate wisdom, it's an instant as far as I can tell but didn't stop at the end of opponent's turn even I had the mana (!?)

this only happens rarely, most of the time works

Same issue. Had mana open but never got priority to cast instant side of Virtue of Loyalty.

In retrospect, the opponent may have had Voice of Victory on the battlefield...

Same issue, it's quite critical bug

Declared blockers with land/mana in reserve to cast Restoration Magic, but never had the chance to cast instant before damage was dealt. I've had similar issues with instants not getting priority before.

Also losing matches due to uncastable instants that should be castable.

I could not cast a one mana white instant spell despite having untapped white mana. It was totally game-losing.

Couldn't cast instants from the second main phase all through opponents phases.
Played on android

Instant not castable during combat phase.

Same issue. Wtf lost me the game hard. Complete ********