See updater_write function here:
https://github.com/pycom/pycom-micropython-sigfox/blob/master/esp32/ftp/updater.c#L122

I suspect that when using the pycom.ota_write, it's possible to write beyond the OTA partition (and into the partitions after it, such as the filesystem one). I don't see any bounds checking in the updater_write function nor in the micropython binding code.

cc @Xykon