I found this post and I am experiencing the same issue.
I am using Quicken Classic Business & Personal, I have ensured I have the latest version and have run the validate and super validate data. The problem remains. I entered my first customer invoice , saved and closed. I then went to the list of all invoices and selected the display form for the invoice, this is when I get the message "some split data lost…" same as in post above. I am unable to delete this invoice, the only way to remove is to restore from backup. I also noticed if I enter a second invoice, it seems to work correctly. I can edit the 2nd invoice, it doesn't matter if it is the same customer or new customer, the edit works as expected and I can delete the 2nd invoice but still not the 1st invoice entered. The 1st invoice entered is trashed with no way to edit or remove without restoring from backup. Any thoughts as to what is going on…