Empty value in Excel cell?

I have a program which receives .xlsx (Excel file) as an input.

Somehow I manage to create some, lets say, "strange empty state" in one of the cells crucial for work of my program. By that, I mean that cell looks perfectly empty and cause program to has behavior A.

However, If I mark that disputable cell, press "delete" button (try to empty content) from keyboard upon it and save document, program has different behavior B.

Any idea which value could be initially in the cell which looks perfectly empty? And how is possible that simple deleting changes content of the cell? Any idea what to do?