Final pass always gets bumped and chews into piece

I’m attempting to cut what is basically a simple circle out of square 6/4 (1.25") white oak stock with a 1/4" downcut spiral bit (though this occurs with a 1/4" straight cut single flute and 1/4" upcut spiral also). At the final (or near final) pass, at the bottom left quarter of the circle the bit catches on the piece and chews it up or, worse, spins it out of its clamps and burns through the center of the piece. I have tried optimizing the feed rate and cut depth and moving the piece over on the board.
When I try a similar cut on thinner stock the same problem occurs, always at the bottom left quarter of the circle. For the thinner stock what seems to help was removing the tab on that quarter and just cutting straight through and let the other three corners hold the piece in place.
The thing that is so confusing is it is always in the exact same spot on the piece regardless of where I place it on the wasteboard. I’ve noticed that one of the last passes the X Carve does seems to be a very shallow skim pass along the cut. Is there a way to remove this from it’s cut? Or has anyone else had a similar problem?

Well, you can control that pass from being so thin with math… total cut depth divided by depth per pass and if the result is a small fraction of a whole number then your final pass will be thin, you can adjust your depth per pass as needed to get a higher remainder value…

The rest of your issue, I have a couple theories but if you could share the project link or would be easier than going back and forth with a bunch of Q&A.

I will also add, that I tried NOT cutting all the way through an leaving 1/32"-1/16" of wood on the bottom and no matter where the final pass lands I have the same issue.

Alright lemme do that

this is without cutting all the way through. Has helped the littlest bit

I am guessing that you are clamping the corners correct?
This project would work best with the blue tape and super glue method and no clamps.
Also is your but capable of cutting 1.25" deep?

I clamp all four corners and start at bottom center of the piece, yes. My 1/4” bits are all 2+ inches long. Would I do better keeping as much shank in the router as possible? I’ve tried to do so but give it enough play that the router chuck doesn’t punch through the dust shoe.

So when I say skim pass, I’m referring to the bit skimming along the inside of the cut and rubbing up ever so slightly onto the piece. For whatever reason on the bottom left it seems to catch very easily on either the piece or the waste, I haven’t been able to discern which.

Yes, but what is the cutting length?
Also I suggest you increase the size of your tabs if you insist on only clamping the corners.

Okay I can increase the waste, I was attempting to limit it since I’m cutting my own stock. Interesting question about the router cut length. It’s a Freud 76-102 which is a carbide length of 1”. I was under the impression that if it’s below a cut line the carbide height wouldn’t matter since I’m only cutting .04” depth per pass. I guess I didn’t see any reason a smooth shank below a cut line would be a problem.