Positions problem

I've got this irritating bug in a match, that when a player plays as f.e a striker. He will be playing defender without I ever setting him as a defender. If I switch him back to striker he will be playing striker for a few minutes but then again he will be switched back to defender after a while.

Not only striker but I got this problem with midfielders and defenders switching too.

It's really annoying when I see my striker defending and make mistakes and get a goal against me.

This problem doesn't seem to occur in the normal 4-4-2 system and some other tactics.

But I got this problem with many other tactics and I would like to know what I can do against it. Am I the only one having this problem?