If you’ve still got HA configured then the VRRP process is still running on the MX, even if the secondary is offline. A primary unit will drop its priority to 75 if it has no uplinks (WAN 1 and WAN 2) available, is this what you are seeing? Do the priority changes coincide with events on your uplinks?
Are they consistently occurring every six hours? Maybe it’s a VRRP timer of some sort kicking in, but you’re correct, there doesn’t seem to be any uplink state changes. The easiest was to solve it is like has been said, remove the HA configuration if you’re not using it. Otherwise you could log a support case and see if they come up with anything.