Hi Olivier,
I tried but didnt succeed in reproducing a similar situation. I was triggered because in the past I had something similar with a datasource MF being called all the time when the page widget it was in was not inside the dataview with the page-parameter. Solved that by moving it inside a dataview.
Only things I can think off
The default cancel button Mendix (the kind that doesn't invoke a microflow) does a rollback and a close page.
I'm not sure, but I can imagine the rollback also refreshes the object (your page parameter) triggering the datasource on your nested dataview.
Have you tried creating your own custom cancel button, explicitly not refreshing the object you rolled back?
I ran into this very issue. The X cancels and rolls back the data on my form that hasn’t been committed yet.