I think this was either a `number of TBW (terabytes written)` or `% of space used` issue, since both got removed by the OS within 9 hours delta and the same usage (couple of months, ~500GB), because they were in a mirror since the beginning. If it were a sensor issue, SMART data should have shown this. Not saying that a sensor issue does not exist, but I doubt that this was my problem (mine don't have a preinstalled heatsink, I prefer to use my own).

I now use a 3-way mirror and am mixing brands.

One very nice thing: the Samsung Pro 990 4TB has the exact same space (down to the byte) as the WD_BLACK SN850X 4TB, so they can be replaced without any issues. This rarely was the case with SSDs and HDDs and probably other NVMes. Looks like they learned.