Animated FBX from Unity… to Blender… to Unity. Blender messes up the rig?

I need to take an existing animated model from Unity, import it into Blender, do some minor adjustments to it and export back to Unity.
The important part is that it should still be able to read its existing Unity animations.
However, the simple act of getting the model into Blender and back into Unity seems to mess it up so its bone structure is now weirded out by the animations - they don’t work properly anymore.
My question is - what are the precise steps to do this correctly? Can it even be done?
My Fbx export settings:

A Unitypackage with my rigged Unity model and its animation that needs to be usable after exporting the model into Blender and back.

I have always found the whole armature export process somewhat convoluted.
It’s hard to tell what you would need in your case, but maybe give deselecting “Apply Transform” a shot.
From the top of my head, I believe having that option checked when exporting from Blender > Unity can flatten/mess up the bone hierarchy - of you hover the mouse over the little warning triangle it will give you more information.