When you were upgrading ROS, did you upgrade routerboot as well? Sometimes there are (tiny) discrepancies between (old) routerboot and modern ROS which are only resolved by upgrading routerboot to modern version (shipped with ROS itself), which has to be followed by reboot of device (sorry for that).

/system routerboard upgrade

only this one?

Did this actually start with upgrade? Or was it present with previous ROS versions (but you did not notice it)? Obvious reason for that happening is CPU frequency way off what router thinks it should be (e.g. 20% slower than it should be). I'm not entirely sure how CPU frequency gets determined at boot time, it's easy if device has (at least semi-precise) RTC and calibration loop can count number of CPU cycles done inside certain time duration. But MT devices (AFAIK, at least most of them) don't have RTC, hence 1.1.1970 at boot and if router doesn't have external source of time (e.g. NTP or MT cloud or whatever), then log timestamps stay in year 1970.

You can't see the real cause of error only by observing timestamps with 1 sec resolution. System time might actually progress uniformly, but display gets rounded off and it could be the rounding error you see when router "doesn't advance the clock". But if this is the real reason, then it would be present with all versions of ROS running on that particular device.