X-T4 low light "flecking" noise, part deux

A few months back I posted a thread about a phenomenon I had noticed with my new (used) X-T4. I had both an X-T2 and X-T3 body lying around at the time that I was able to test as well - unfortunately have sold them since, so will make do with the images I snagged that night. The topic slipped my mind for a few months until I was sorting through a hard drive and noticed the test pictures. I never really got a conclusive answer on the first go-round.

It basically boiled down to this: at high ISOs, there appears to be a high amount of noise. Well, duh, right? The weird part is that the "noise" manifests as white "flecks," rectangular shaped, that almost look like dead pixels. X-T4 seems to have the most, with X-T2 close behind and X-T3 being notably cleaner than both of the others. All of these shots are taken at ISO 12800 and are heavily contrast boosted to see the effect plainly - see the filenames for which body is which: https://imgur.com/a/3JxW4y9

I am still very much stumped on this issue. It doesn't seem to affect normal daytime shooting conditions, and I haven't had time yet to test it in a more realistic low-light scene. It doesn't become an issue until ISO 6400, and only becomes unusably bad at ISO 12800, which is already going to be "noisy," so I guess my question is two pronged:

1) if it's just high-ISO noise, why does it present as bright white on a dark scene?

2) Why is it so much worse on the X-T4 than the X-T3?