Comcast recently sent us a couple of new cable boxes, and these have the XR2 remote.  I also own a XR11 remote which I believe is shipped with some X1 systems.  So I'm pretty sure we know which signals need to be sent.

XMP signals have a toggle which is set to 0 for the first frame of a signal and set to 8 for subsequent frames.  However, The Pronto Hex in the link I provided above isn't quite correct: it simply repeats the first frame with toggle of zero.  So your X1 is seeing what it thinks is two button presses.

IrScrutinizer can provide the correct Pronto Hex.  But there is an additional wrinkle.  XMP in its full form sends 16 bits of function data, but in practice most signals are sent with 8 of the 16 bits set to zero.  If the function is encoded in the first 8 bits, and the second 8 bits are set to zero, we describe that signal as XMP-1. That's the one you want.  So select XMP-1 and then fill in the other boxes as you described above.

BTW, it isn't obvious, but you can generate XMP-1 using the XMP protocol in IrScrutinizer by entering e.g. 0x0300 instead of 3.