Maybe because the guy had his best gear in move 4 and he is not aware of it. If the difference between his real move 4 initiative and your real move 3 is higher than 10k then he moves before. Lets say in matsuri his move 4 has the stats of his equipped move 1 and owns 18000 initiative, if your real move 3 has 6000 initiative then he is able to interrupt you.

What happened is that your mystery was delayed. You were not actually interrupted but, server side, your ninja died before the server received you mystery input so, even if your ui showed you the icon of your mystery in truth the mystery was not calculated in.

Can you tell us how they were placed on the field your and his ninjas? I begin to think maybe the enemy main was not in 4th prompt move (moreover because there are not many meta lineups that put mb with acupuncture seal in move 4... )

team was placed like this (masked man in place of iruka, i don't have masked man lol)

Did the lighting main who used sealing slash use any sort of summon mystery like Pakkun or Shiba when interrupting you?

Can you tell us how they were placed on the field your and his ninjas? I begin to think maybe the enemy main was not in 4th prompt move (moreover because there are not many meta lineups that put mb with acupuncture seal in move 4... )

There is one thing I noticed about summon mysteries. Once the main has clicked his mystery he will use his summon mystery before that. The thing is, the time span in where the main does his summon mystery is a point of no return. You can't under any cir*stances interrupt the main's mystery during the time span in which he does the summon mystery. When the enemy activates a mystery during that time span of no return it won't go off till the main has done his mystery. That being said, if your position 1 activates a mystery while the other main is doing his summon mystery, you won't go first but the main will. That can result in the 4th position acupuncturing and interrupting the position 1 character. In your scenario the enemy probably just wanted to do damage against your Kushina but your Kushina mystery was activated by the game during the time span in which the main did his summon mystery, resulting in Kushina being interrupted. I think I know why that is happening. The game doesn't register the summon mystery as an action before the mystery which can be used for reaction time (e.g. Mabui's 40 chakra passive) but instead it counts as part of the actual mystery. That's my theory at least.

why cant it just be simple?
If you have higher initiative, you go first

no bs, simple as that.

bc of delay, i couldnt interrupt water main and since i was fighting cancer team, that changed the match.

Ok, i understood what happened - i think - you just had used kushina mystery in that round, somehow you endured 6 hits with her and the cd got reset so you clicked or the auto did it for you on her mystery again, but since the mystery couldn't go off again in that round was delayed to the following round. In that case can be interrupted even by slower mysteries. Every dance of impetus user know about this.