STudy the logs to see if anything appears. /var/log/errors.log and /var/log/everything.log, although I do not think everything is really in everything log, almost everything appears there as well in specific logs.
You might see disconnect and something just before it happens, could also change your log level to more detailed with adding to the same line as everything else in /boot/cmdline.txt, loglevel=6 as default is lower, loglevel=4. Then issue sync command, poweroff and reboot.
Logs will grow fast but maybe they might reveal something helpful if you increase detail temporarily.
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