Excel Heading Row Disappearing

I'm using MS Office 365 Pro Plus. When I open the excel file, I can initially see the heading row (A, B, C, etc.) but then quickly disappear within seconds. However, the header column (1, 2, 3, etc.) is still there.

When I zoom in (like 200%+), I can see the heading row is there but it's not visible with normal 100% Zoom Level.

I have tried:
•Changing the font type and size for alphabet and numbers displayed on heading row and column

•Checking/Unchecking View> Headings
•Hiding the formula bar

But, no luck and seems the issue isn't related to them at all.

Any help would be greatly appreciated as I'm having a hard time working without header row. Thanks heap for your time.

Additional Info: Sometimes it appears and disappear when the second excel file is opened.