The documentation of the library states that, do not perform any opeartions before or after adding a sheet in its "Things to Note When Using robotframework-excellibrary" section:
When using the keyword Add New Sheet the user cannot perform any functions before or after this keyword on the currently open workbook. The changes that other keywords make will not be saved when the keyword Add New Sheet is used.
To overcome this, in the Run Keyword If fragment, you have to also save the file, and re-load it