Hello!
I think, your issue is no real issue, but a layout glitch, which is easily to fix.
First, the reason:
You have different breakpoint and it is necessary to place all your elements correctly(!) breakpoint-wise, because Muse can’t know, where you want these elements to be placed. The sense of breakpoints is exactly this: Being able to place elements in each breakpoint independently at different positions and different sizes. (This can be done "half-automatically, when you place your elements at the widest breakpoint and create the other breakpoints afterwards. In this case, Muse tries to place and scale the elements relatively to their position/size of the widest breakpoint)
Solution: Place the elements breakpoint-wise where you want them to be, and you should be done.
Look at this little screencast, and you will see the problem (I outlined the "jumping" element in red:
Here you can find a modified version of your file, which should work as expected:
https://www.dropbox.com/s/cl42wr2nkz5dqu0/Essai_debbuging_mod.muse?dl=1
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