Strange Rigging Issue

I've been working on rigging a model for the past couple of weeks on stream and everything has been working fine. Until this past Thursday, where after 45 minutes of working just fine on stream, this started happening:

Posed hand with full weight assigned to bone Same hand, clicking 100% weight/strength/constant falloff

Yes, I'm painting 100% weight to a bone that already has 100% weight. On Thursday I encountered another oddity where doing that actually only painted 50% weight wherever I clicked, like I had another bone selected, but I made sure I had only the one bone, multiple times, by selecting it from the outliner specifically.

I'm not sure what happened all of a sudden, but it only seems to be happening with my hands, my forearms, and oddly half of my biceps. The rest of the body is perfectly fine! Legs, torso, head, all good in any pose.

There's no double bones, no extra weight assignments from another bone (I made sure to select all bones but deselect the specific bone, clear weights, then select the specific hand bone and try repainting), and as I said the weights were working perfectly fine before (as you can see, the hand is in fact weighted and posed, and distorting as per the thumb). I'm not sure what caused this to suddenly be a thing.

In this specific pair of images I'm doing what I call "the rigging dance" pose wise to stress body positions and make blending weights easier. This is one of the keyframes for the arm and hand in a downward, hand bent back pose.