@lance.carocci I am experiencing the same behaviour:
Switching from fusion to e.g OSX mail, safari or calendar, the fusion browser stays a float and is active.
Switching between mail, safari and calendar triggers the same behaviour on the first time the program is activated after fusion had been left in the background. Usually after this fusion keeps quiet.
But: minimising and maximising any other application window has the same effect constantly. Fusion pops into the foreground with the browser menu activated.
This is literally stealing my focus and is tremendously disturbing to my workflow.
A workaround that fixes this issue is to close the browser window in fusion 360:

This way fusion 360 does stay in the background solid.
I happen to tab switch back and forth between applications a lot during CAD work. Researching materials, availability, dimensions, looking up specs in safari, emails, photos, you get the picture. This bug is a serious issue to anyone who professionally works with Autodesk fusion 360 on a Mac. And for a paid application it is just downright unacceptable and should be given top level priority.
Reproduced on different iMacs with mac OS Big Sur (all iterations thus far).
Main specs:
iMac Pro (2017)
OSX 11.4