ok, so you using a battery charger that is ment for lifepo4 but using it on a lead acid, well thats you issue, lifepo4 charge at higher voltage compared to lead,
i would change the charger for one ment for the designed battery.
if i's a universal charger (RDS-6012 or victron 30amp charger) then you nede to adjust the charging specs to match lead acid.
darkspr1te
You are answering based on previous reply which was not made by me.

It is a lifepo4 battery.
The solar charge controller is not 'made' for lifepo4 out the box however it is able to be programmed using custom profiles. I was given the settings by a more experienced person on another forum which I am no longer on for a good lifepo4 charge profile.
I actually realized something I was missing.
I forgot about resting and charging voltages.
I am thinking that probably the reason is that the voltage is raised above resting baseline, due to solar charge, when I am looking and seeing the light flashing. Something to do with that I imagine.
Anyway not a big issue really. The mppt knows what to do it seems, having worked for over a year.
I don't think the specific device matters. My question is a general one in that how would mppt chargers know when a battery is full. Is it based on the voltage at the battery posts?
I can tell you the mppt anyway. It is epever tracer bn 4215.
Manual.