In recent update of MyHeritage's backend software, they blocked non-Premium/PremiumPlus accounts from syncing with the online tree. As side-effect, this resulted in corrupted data being received if user tries to use Smart Match lookup.
If user tries to match existing persons on offline tree with online trees, birth and death dates are synced, but the field name is changed to blank, which causes further editing to corrupt the entry. There is two workarounds for this corruption:
Manually copy the birth and death dates from bad fields to correct fields. This will not work if you remove or edit the corrupt fields before copying the field contents to correct fields. The software will try to restore last backup if it detects corruption.
Do not sync birth and death date fields and instead copy just the name fields. This only works for private profiles when you use "Replace all details" option in "Step-by-step merge" of Smart Match. After you have created the entry locally, go back to the "Smart Match Merge Utility" dialog and select the local entry and choose "Edit details". Make sure you are not covering the online entries with the "Edit Details" dialog, so you can copy the birth date, birth place and death date fields to the "Edit Details" dialog. Before closing the dialog, open the "Facts" tab and make sure all the duplicate field names have same contents. Do not try to delete any of the duplicate fields. Close the dialog.