Your Problem is almost Certainly A Broken Bumper assembly. The only thing that is unfortunate is that the 1708 only technically has 1 Bumper assembly for Both buttons, as they are connected together by a thin plastic piece. (it’s a pretty crappy design too, they rely on bending of the plastic for the button’s behavior.)
I’ve gone through 2 assemblies already (just installed my 3rd) as I use RB heavily in a few games.
'''The first time it happened when I opened the controller I didn’t notice that the button assembly was cracked on the cross connecting piece that’s hidden underneath the center retaining clip''' (the part with the Bluetooth button hole in it). At a glace it all looked good but the button still felt squishy and required a hard press to activate and didn’t seem to return/click like the left one.
You say you already opened the controller? I’d carefully pop off the center piece. if you haven’t already And check the whole assembly for fatigue, or cracks. There are 2 small pegs holding in on that would be on the Front of the controller. you just have to lightly lift the center piece off the 2 small pegs
The odds of the tactile switch being bad are pretty slim, but I suppose it could happen. But honestly, all the plastic pieces have more of a chance of being broken than anything else.