Flashing CEL low speeds/idle on EV only

I recently replaced the engine coolant pump (not hybrid), after I was getting a p261b code. I drained the coolant before then put it back in after the replacement. Put into maintenance mode and test drive a bit. The code went away and it drove well at first. Then, I started getting a flashing CEL that would come and go. After some troubleshooting i noticed that it would only start flashing on EV only mode either charging or not charging the battery, didn’t matter. I put it into maintenance mode again and I noticed that while it was on maintenance mode, it would only start flashing when I turned on/off lights, opened/closed doors/hatch, locked/unlocked doors, anything electronic it would start flashing. When the engine was on, the CEL would flash and the engine would rev slightly at the same time. When the engine was off, the CEL would just flash when I put more load on the electronics system. If the load was not reduced, the flashing CEL would either stay on longer or not go away (just continuously flashing).

also I disconnected the battery system before changing engine coolant pump.

no codes. Just flashing CEL.

I cannot figure out what it might be. Seems strange to me that the electronics system is tripping the flashing CEL. The 12V battery health is good. It was manufactured in 4/22.

 

i replaced the engine coolant pump, not the hybrid inverter pump. this is a 2012 prius II.
at first i thought it was air in system, but the CEL only flashes when i turn on/off electronics and only when engine is off ie EV-only mode. I can verify this bc after i turn off the car, the monitor displays 99.9 mpg. why would the CEL flash when the engine is off if i replaced the engine coolant pump?
the car was not doing this before, but the car was sitting for about two weeks, and before i purchased it, it was sitting on two different lots for over 4 months. i am suspecting it is the hybrid battery. i would appreciate your thoughts/opinions on that suspicion, or on whatever else it might be.

 

after i removed the water pump i tested the rotor - it did not spin as well as the new one that i purchased and there were iron filings in the rotor cavity. after i replaced the pump, the code went away and the engine did not sound as loud/rumbling as before which started happening a couple trips before the code was triggered.