ISBLANK works sometimes but not others

Ideally I’d like to do this: in cell G5 I have =IF(C5="","’,E510), works only if C5 contains data, although on other sheets works fine. So I cut and paste, change formatting, etc still won’t work on some sheets. I then tried =IF(ISBLANK(C5),"",E510) with the same result. What am I doing wrong here?

Hard to tell without sample problematic workbook

And please edit your question don’t add an answer if it is not. Adding the LibreOffice version would be fine.

Ideally I’d like to do this: in cell G5 I have =IF(C5="","’,E5 * 10), works only if C5 contains data, although on other sheets works fine. So I cut and paste, change formatting, etc still won’t work on some sheets. I then tried =IF(ISBLANK(C5),"",E5 * 10) with the same result. What am I doing wrong here? I reposted this to get the correct formulas in the question.
I have what I would call more of a work-around than a solution. And that’s to use: IF(C5="",0,E5 * 10). While the original works on some sheets, this seems to work anywhere. Reformatting has no effect.