SymmetricDS uses a separate channel for initial load batches by default. When initial loading in a active environment, these FK violations can occur temporarily by design.
When the initial load channel runs into a FK violation the channel is suspended. Your data channel containing the missing PK will start to sync. Once the data channel is complete, the initial load channel will attempt again to sync. This time there won't be an error because the required PK is now available on the target.