Don't know if related, but before this plugin was available, when I first got my accelerometer, I wrote some gcode to collect accelerometer data for a number of moves along x, y axes and diagonals (8 M956 measurements per run of my macro).
After running the macro successfully several times, RRF (a 3.4 beta at the time) crashed (during one of the M956 commands, I think) and I had to hard reset the board. I think I even had to cut 5V power (i.e. unplug the USB between my RPi and my Duet 2).
Like in your case, this only happened after many measurements. Also, like you, my accelerometer is connected to the Duex (spi.cs8+spi.cs7 in my case). But my wiring was a cat5 cable with an IDC connector on the duet end and 5 pin + 2 pin JST XH connectors on the accelerometer end (Adafruit LIS3DH).
On that day, I could repro the problem a couple times. But by the time I was going to report the problem and collect some debug logs, it stopped reproducing for me. I haven't tried the new plugin yet.