Not sure if this is a bug or what, but looks like even though Unstoppable Fusion says 'any fusion rifle' it means 'as long as you have a fusion rifle equipped, your Linear Fusions will also have Unstoppable' Screenshots for proof
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Update: looks like it still procs when you just have a linear equipped, but nothing in killfeed or on the gun to say that unstoppable is active after assuming down sights for a bit.