After successful initial and delta loads in CRM no BP updates in CRM replicate in ECC.
After creating a BP in ECC via transaction FPP1 (role MKK, account group CUS4, BUPA_MAIN) and replicating in CRM, a BP change in CRM does not replicate back to ECC.
The BDoc message shows that ECC does not have such Business Partner even though it came from ECC. One potential problem I see is the fact that the middleware trace is showing object CUSTOMER_MAIN. I would expect BUPA_MAIN in order to point to BUT000 instead of KNA1.
I also noticed that the extended BDoc message body is showing a BP without leading zeros whereas all other structures don't.