5th gear yeeted itself? Or clutch?

I was recently out riding with a bunch of buddies on a 2012 YZ450F. I have a timber sled kit on it and was riding 50 plus miles one way to a camp in the snow. Everything was going absolutely fantastic and I had no mechanical complaints whatsoever the whole season. However, I noticed when going high revs / speed in 5th gear the bike would hiccup and slip like a clutch slipping or chain skipping. You could also hear a soft clunk and engine revs increasing for a quarter of a second. I lowered the revs / speed so I wouldn't cause any more damage but slowly over the course of an hour or so the clunking was occurring at lower and lower speeds/ strain. I couldn't really stop or turn around cuz I was already too far and had to press on. Eventually, I couldn't really use 5th gear at all. Even at low speeds it's clunking pretty miserably and hiccuping badly. The weird thing is, no other gears have any issues whatsoever. All of them run smooth as butter and I have absolutely no complaints. Furthermore, I did mess with clutch adjustment at both the adjusting nut by the case and the one on the handlebars, varying the play in the clutch. Neither of them change the clunking at all. And again, all other gears run fantastic, even the top end of fourth under high strain. I also did the test of putting it in fifth gear with the clutch in and revving it to see if the bike would move. Nothing. I've never messed with transmissions and my clutch experience is limited, does anyone know what this sounds like / what I need to look at and repair. All I need to know is what issues it could be and how to look for them.