A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Does this happen in one particular file or all Excel files?
You may disable the add-ins for Excel and then check if the borders work fine. In Excel, click on File > Options > Add-Ins > Manage COM Add-ins > Go (at the bottom) > uncheck all the add-ins and click OK > close & restart Excel.
If it’s an issue with one file, you may try repairing the file using the Open & Repair option and see if it helps. Open Excel > click on File > Open > browse & select the concerned Excel file > click the drop down menu and select Open & Repair at the bottom right corner.