Non-prompt mystery means it is casted when it's that certain ninja turn to attack with standard move - that is determinated by position of that ninja in team- number shown above the ninjas heads while you set up your lineup. That doesn't mean it is the last action of the round. And if someone can time it perfectly and/or get a bit of "help" from the side of lag between players- he c*e the mystery just in perfect time of certain ninja time to attack so the icon wont even show to you or will show for so short you wont even have time to react. It's not bug but more of great timing and sometimes lag.

You can still block it when your timing and predictions of actions of your opponent are also great- let's say he have Gaara on move 2 position. His ninja on position 1 did his standard, then your on position 1 do his. Now it will be time for Gaara to use standard and if you cast Iruka, for example, then you might just be in right time to block his mystery- but if you will wait for the icon to show- then you wont make it if your opponent click it just in that right moment.

And again- non-prompt doesnt mean it will be last action of the round. If said opponent would click Gaara's mystery on start of round- but his Gaara is on position 2, 3 or 4 to move- then you have all the time in the world to block it, because he will use it only at his turn to move.

Also, if you click on mystery that is non-prompt after that ninja, ex. Gaara did his standard attack already- then that mystery will be casted in next round when it's his time for standard again.