[readmodifywrite]

It really seems like it has to be something like that. The problem is there is no detail in the docs and no status bits in the chip. There's no way to know when the auto-cal runs.

One of the several things I did to eliminate the problem was to disable the auto-cal during a UART reception (the STM32 is the bus master so it knows when it will be receiving) and re-enable it when it is finished. It absolutely confirmed that is the glitch, but I don't think I'll ever get a true why unless an ST engineer wants to chime in!