1. Check that the column widths and row heights are the same for the source cells that were merged compared to the cells you are unmerging them to.
2. Ensure any adjacent cells are not set to overflow, wrap or truncate, otherwise text will appear misaligned when unmerged.
3. Make sure the orientation for the cells is set to the same orientation orient before unmerging or else text can appear misaligned.
4. Try to adjust the cell formatting such as font size and line spacing to make sure it appears consistent when unmerged.