See this thread with some details on similar behavior myself and others are seeing: Orion XS 1400s fluctuating charge output under inverter load
The running theory from Allen Jones (video linked from the Facebook post) who did extensive testing is that this is caused by AC ripple and increasing the size of the battery bank can improve it. I’m seeing this on a 600ah AGM bank. Sound like getting to around 8-900 helps resolve it. That said there’s reports of this behavior on 1300ah banks…
Try limiting the Orion output to 45A to see if that makes any difference.
Let is know if you have any success in resolving.