A sheet named "undefined" relentlessly appearing in my Google spreadsheet

Question

I'm a long-time Google Sheets user, and out of nowhere, one of my spreadsheets started getting a weird issue: a sheet named "undefined" keeps appearing relentlessly, copying the raw values of one of the previously created sheets, even though there's absolutely no error whatsoever. I have other spreadsheets featuring the very same named functions and app scripts, and they feature no issue at all. I tried deleting the sheet that is being copied and make a new one, didn't work. I tried making a brand new copy of the spreadsheet, the issue still appears. I don't know what to do to solve this, since it's too specific.