Documentation for the Sophos Intercept X Mobile QR code scanner should probably include something about the limitation of the QR code scanner. I created a 61x61 plain text QR Code using an on-line service, and scanned it three times in a row, the three results were:
The actual text doesn't even contain those values, so I installed a GAMMAPLAY QR Scanner from the Google Play store. It scanned properly.
By experimenting, I found the problem to be that your scanner can't handle a QR code that fills more than about half the height of the scan window, and it didn't matter if I was scanning the QR code displayed 15 cm square on a monitor or printed 14½ cm square on paper. If I held the phone far enough away from the print, it read it properly; too close and it gave me random 8-digit numbers. (In case the problem has something to do with the camera, I'm using a new Samsung Galaxy S8.) Here are 3 identical 81x81 test codes at 200PPI, 300PPI and 600PPI. Even the 200 PPI one doesn't scan properly if I hold the camera too close to it, but far enough so the entire code fits in the scan window.