2 x Deye 5kW in parallel.The slave constantly feeds 300 plus W back to grid.I have changed the master to slave,slave to master.I have moved the CT.I have upgraded the inverters downgraded the inverters.I am stumped.This only happens on export to load.

Interesting. So not the inverter, just the role (slave).

I have a single 5K Sunsynk, and when generating PV, it over-reports by about 300W. Consistently. I have leaned to live with it, because I am not going to uninstall it to send it in for RMA, it will leave family members without an inverter for an extended period of time. Pretty unhappy about it because Sunsynk reneged on their policy of providing a loaner for repairs under warranty.

Your issue is more serious than mine. Have you raised the problem with SS Support? What do they say? RMA?

2x Deye.I going to attempt something at the client then report back