The two media machines, also running Trixie with the same filesystem mounted, are unaffected.
The two failed machines both have Supermicro X12SPI-TF mainboards. One has a recently acquired secondhand Xeon 8362, the other a 5320 that had previously lived in my VM server for the last few years and had no problems.
RAM on both is recently installed, but hand-me-downs from other machines with no errors during POST or operation. Everything is ECC.