Sunsynk voltage drop when charging

I have two parallel 5kva Sunsynks, running with two 5.2kva Dyness batteries.

Dyness advised that the batteries should charge at 2x50A.

Whenever the batteries charge from the grid at the 100A, the reported voltage drops to 187V (from 200-210V typically reported by the inverter when not charging the batteries). The low voltage then seems to trigger the inverter disconnecting the grid supply, and running off battery only (before retrying 60s later).

When I reduce the battery loading current to 50A, the voltage drops to a reported 190V, but at least the inverter continues to use the grid to charge the batteries.

However, even at 190V, my lights are dim and flickering, and my computer equipment UPS kicks in.

Is this type of voltage drop normal? Is it a possible fault of the inverter/charging system? Or an installation issue? Settings issue?Battery problem?

Any guidance would be much appreciated….

thanks