Q: How to get rid of (bad) filename suggestions (text auto fill) in Save dialog?
A: This is not caused by IrfanView. Many objects in the File dialogs (open/save) are managed by Windows (File Explorer). This issue is caused by the "AutoComplete" option for File Explorer. You can disable it in "Internet Properties" dialog: search for "internet options" or "autocomplete" in Windows Settings, open the dialog, go to "Advanced" tab. Scroll down to "Use inline AutoComplete in File Explorer", uncheck it and press OK.