Seem to be having a similar issue. It feels to be related to the latest AT&T Uverse update. Does not appear that Sling updated after. This other thread says to do a hard reset but have not tried. Let me know if that is the prob solver.
I have a slingbox m1 connected to a wireless AT&T Uverse cable box (I can't remember which one off the top of my head at the moment since it it as my parent's house). It used to work perfectly but recently, when I input the channel into the slingbox app on an apple iPhone 6 plus running iOS 10.1 such as 1004 or 1772 it transmits as 104 or 172 skipping the third digit. I have reconfigured the network, outputs, reset the router, and unplugged everything to no avail. Has anyone else had this type of problem and if so, how did they fix it?