I've had this same problem. I have a sneaking su*ion it stems from the whole "que" system.

If you notice, sometimes if you used a prompt mystery a bit later than expected - your ninjas will standard attack or even enemy will standard attack. Such as, I let enemy Roshi go first, then I use my Roshi directly afterwards, however my Roshi and enemy Roshi both do a standard attack before the system acknowledges my roshi's mystery. So So the system views it as like a queued system:
1. Enemy Roshi does mystery

(Then you hit mystery)

2. Enemy Roshi does Standard
3. Ally Roshi does standard.

4. Ally Roshi does mystery

This is both bad and good at the same time, it is easily abuse able, as I do all the time. The moment a mystery goes off, all the chase combos are already basically being applied. So if an Enemy Roshi targets my Roshi to try and cancel my Roshi mystery, but I wait until mid chase combo to use the mystery, the system does not recognize it as being interrupted as it wasn't being cast when the first hit in the chase combo was applied.

I feel like this is where the issue with interrupt people stems from and why it appears that position 4 people almost instantly use their mystery even faster than a position 1 (When trying to interrupt that is, by no means will a position 4 ever go before a position if both used prompts at the same time)