I can’t say for sure.
How many amps are you pulling from it?
3500w from 12v is quite a load -that would be 291Amps. So your bank as mentioned is way too small for the job at hand.
I am not surprised the voltage drops so rapidly.
If you are drawing 50A (600watts), that battery will reach 50% DOD in under an hour.
It is only a 1.2kWh battery. At the very most will take 2 hours to reach 100% DOD.