NOTE : This is a bug on which the engineering team is currently working on . The fix may be part of 2020 R2 SP1 release (11th June 2020 PROD).
Until then please use this as a workaround.
We wont be able to transfer the entitlement back to source account, when there is an active Split.
Split the full quantity back to source account and not transfer to source account.
Activation code will change if the property 'TransferSplitEnabled.TransferOrderActivationCodeToTarget = True'.