Unable to merge because sub-entity will be parented differently. You can disable the parent check prior to execution as part of Merge dialog.
Enable Parent Check
This is interesting.
If this option is checked and the records have different parent Accounts, then on merging following error will be thrown.
if you tried to merge two accounts or contacts that had different parent accounts, you'd see a warning indicating this and checking you were happy with the change. After the change to asynchronous merging, the merge would appear to work when you clicked OK in the merge window, but the background system job would fail.
This new setting allows you to pre-emptively agree to any parenting changes during the merge so that the asynchronous merging background job can complete successfully.