Definitely the issue. My new master cylinder and slave cylinder that I have sitting around fit much better than the ones that're installed... basically I'm at risk of the pedal not working if the plastic piece wiggles just the right way and blocks flow. I was trying to find a way to cable tie the assembly in but man it's tight down there. Hard to see what I can wrap around to pull it as linearly as possible inward to the female portion. Would be a decent band aid fix.
My new slave cylinder nor my new master cylinder came with the spring clip/staple thing that holds them together. The one that's installed looks like this, I think it might have just been rigged up by the shop. It's easy for it to slide off and fall out, and doesn't hold well:
A picture from a slave cylinder on Amazon shows this, which is a much different shape:
So yeah, this is what it's supposed to be:
Probably thicker and holds it better.