Why does my lens aperture stop working a few hours after attaching the lens?

I recently purchased a used Sigma 105mm f2.8 DG OS Macro lens, and when I tested it during the purchase everything worked fine. A few hours later when I tried it again, I noticed that my pictures were really overexposed at anything other than f2.8 (the min is f22).

I detached and reattached the lens, and then all my pictures were fine again, properly exposed, and I verified that the aperture blades were indeed working. I turned the camera off and on and it was still fine. A few hours later, I tried again, and the pictures were again overexposed aside from f2.8, and the blades were not moving.

I’ve been able to replicate this several times now, and I’m not sure what could be causing this – the blades are not oily, and they clearly work for a period of time after the lens is attached.

For reference, I have a Canon EOS M50, with a third party adapter for non-M-series lenses. The adapter works great for the other non-M-series lenses I own (one Canon lens and one Yonguo). Something peculiar I noticed is that the Sigma lens is very difficult to attach and take off compared to my other two lenses, I really have to give it a good twist when adding/removing it from the adapter.

Any insights or advice would be greatly appreciated!