Having seen similar problems when doing sysupgrade on the devices (mainly MT76-based) I maintain, following problems/ideas:
- I have devices using swap on MMC. Needs to be explicitly disabled before upgrade.
Which gave me reason to dismount all mass storage, if any. - In the past there were issues when triggering the sysupgrade via wifi.
Being always the very last one, suspecting hardware problems, in your special cases I could imagine some type of short power dip when writing the image. In case the (lowest cost) internal power supply of the router is "at its limits", writing the image, needing more current, could trip it for very short period of time. And then the reboot, as power stable again. I had such an issue when using internal SSDs or modems.