Joachim,
Glad that you got it sorted out in your Project.
I have not encountered an issue with Saved versions of my PS Track Mattes (or altered versions), but also do not recall having to make any changes after the creation - so maybe just never experienced what you saw?
About half the time, I create my Track Mattes with Titler, and the other half, they are created in PS, and just Imported. To date, they have worked, as expected for me.
Good luck, and thanks for sharing the cause, plus the good news that you got things working correctly.
Hunt