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.
Oder per regedit: Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\AutoComplete : Append Completion : no