Okay so there's two separate things. First, beast's claw doesn't stack with marked prey. I have the bear standing off doing nothing in case there was something with stalker's link (there isn't).
After you cast marked prey, it shows up in the attack logs as marked prey until you land a beast's claw, then it shows two beast's claws like here. The marked prey bonus is still in effect, but it is mislabeled now.
Once the real beast's claw gets to +20, the first one that is a mislabled marked prey disappears. Here you can see there is no marked prey but it shows in Dorudugan's tooltip
So that's the first thing, beast's claw seems to replace marked prey once it gets maxed out, and recasting marked prey doesn't bring it back either.
The second thing is switching weapons removing the beast's claw effect. This one I could not reproduce in about 10 minutes of trying. But I'm 99% sure I didn't imagine it earlier, so I'd wager there's something funky with the ability. May mess with it more later see if it happens again.
Okay this is harder to show in a screenshot but I just loaded the single class ranger, started combat, cast marked prey, shadowed hunters, fired a couple arrows, switched to a generic legendary mace x 2, used hunter's fang, and it instantly went to +10, as you can see below if you squint through the log you see this is the first time I use hunter's fang.
I then tried switching to the bow and it correctly shows as +2 from the second strike of hunter's fang hitting. I can't reproduce the issue I had earlier with beast's claw / hunter's fang not transitioning on weapon switch, and the order I did things this time, marked prey still shows up even when I get to +20. Very strange...
Just to be sure I loaded the wanderer build again. I attacked several times with one weapon getting a few stacks of beast's claw, then switched to unarmed and it was gone but came back in the correct amount when I used beast's claw again. And when the stack got to 20 marked prey was gone again. Hmm. I may look into this more later but all I can really do is try various orders of doing things and seeing what happens. Definitely not working right though.