I've always had this issues and never thought asking. Anyway to prevent this?
My Launching Opus > Explorer Replacements = Recommended.
For example =Hyperlink("E:\downloads") opens on file explorer.
Probably something internal to Excel that only Microsoft could fix, although you might be able to work around it using a VBA macro or similar which opened the folder correctly.
Works fine here with Excel 2019. I don't remember changing anything in Excel. Opus is set to max. integration.