Bug in CopyData appending to partitioned table

 I can create a lakehouse partitioned table by selecting Overwrite in the destination options and choosing a column for the partition (simplified for this post):

Result:

The behaviour I expected for subsequent runs was one of the following:

1. Keep as Overwrite - a partition with the same ID has data overwritten, but new partitions are created for new IDs, OR
2. Change to Append - a partition with the same ID has data appended, but new partitions are created for new IDs.

Neither of these happens - 1) just overwrites everything and 2) gives an error - so how are you supposed to update a partitioned table? This is the error for Append:

"ErrorCode=TableActionNotSupportAppendToPartitionedTable,

'Type=Microsoft.DataTransfer.Common.Shared.HybridDeliveryException,

Message=Table action 'Append' is not supported for writing into existing partitioned table."