Beginning lines of a gcode file are overwritten by the BIN file

This is a new trick for my 1 1/2 year old Ender 3 Pro with 8bit 1.1.5 board and Marlin 1.1.8.
The print will start with the skirt - bypassing the heat-up lines and the purge lines (I always preheat so the hot end and bed are hot) I get messages from the printer that say:
open failed, File: .
Write power off file failed.
When I pull the SD card and open the gcode file it looks like this:



I used M502 for a factory reset (and M500 afterward) and the problem remains. There is a BIN file on the SD card, but the data in it is from a print a few days ago. There have been 4 prints since.

I'm stuck. As a work-around I'll try adding empty comment lines manually to every gcode file but that's a pain and the power off safety function doesn't work. Any thoughts or help would be appreciated.

Thanks,
Greg