XLTX files are corrupted if not opened from within Excel
XLTX files open from within the Excel iOS app work as expected (open file, it creates a Book1 file that you can Save As a new XLSX file). If you open the XLTX file from outside the Excel app (iOS Files app or 3rd party storage apps (One Drive, Egnyte, Dropbox, etc.)) it does not create the Book1 file, it remains whatever the template was named and only gives you the option to Save A Copy. If you Save A Copy, it will save it as an XLSX file, but the file will be corrupted and unopenable. Renaming the file to the XLTX extension will allow the file to open again.