Steps to reproduce.

1. Create Content Type with Matrix field (min. columns set to 3).
2. Go to create Content form.
3. Fill Matrix field with data (do not add any new rows).
4. Remove non-last row (ie. first or second).
5. Add empty row.
6. Try to save (trigger validation).

Result:
Value of last non-empty row is lost.

Expected result:
Values remain unaffected by failed validation.