Generally, a flat tube with no obvious reason that shows up just after a tire service is due to a pinched tube. Either tube pinched between the rim and tire bead, or the edge of the tannus and tire. The tube has to be carefully spread inside the tire, very difficult to do when dealing with a tannus insert.
The LBS I worked at would install everything carefully, then with very low pressure 2-5lbs in the tire squeeze by hand the tire all the way around to work kinks out of the tube, let the air out, work it again around the tire, low pressure back in, work the tire again sometimes repeating the process as many as 10 times increasing pressure each time a little to prevent punch flats with Tannus inserts. A real pain and why the LBS charges extra.