For completeness - Another possibility for a fairly old device like this is a flash failure, where a write "succeeds" but never actually writes.
You might find the same condition on a SD card that has been used a lot in a camera, or a dashcam or any kind of device that records and overwrites old files a lot.
If the flash is removable/replaceable then that's a decent fix. If the flash is soldered(sodded) to the board then you're up for replacing the flash chips. Or scour ebay for a repalcement device.
If this router is only used for labs, then even with non-permanent flash it can still perform as a useful learning tool. Just don't turn it off or reload it, and consider setting up something like rancid or oxidized to fetch its config periodically as a backup. But don't use such a device in prod without seriously considering your alternatives.