The PM is confused about what it is connected to IMO. You can have low batts (low resistance) but a bad (loose)connection in the wiring (high resistance) between the PM and batts so the PM thinks it is dealing with a battery with high resistance (nearly full) so it goes straight into Absorb or Float except it didn't do its 15 min at boost. (Note that could even be in one of the PM's DC output set-screw type terminals)
EDIT, I also hope that you have fat short wires to the batts from the PM or else it won't do its full amps with long too-thin wires. I have #1 gauge 18" from my 100 amper to the batts. You should have at least #4 no more than say 15" for the 75 amper IMO. Any longer you need fatter wire IMO. (Sorry, I don't remember if you posted about yours earlier.)
When the batts were actually near full, disguising that bad wiring connection high resistance,the PM did its 15 min and then went into Absorb then Float, or at least that is my initial notion, which could turn out to be all wrong :)
Before blaming the PM, maybe haul out a pair of low batts and with nothing on them except the series link, attach the PM to the batt pair's load -taking pos and neg terminals and see if the PM behaves.
If it does, then go back and find what's wrong with your regular set-up for wiring and maybe loose connections.
