I had this happen once before, and to the same channel I believe. My ABC affiliate (WTAE 504) has completely disappeared from any of the guide listings on the Channels Android Client on my Nvidia Shield. I am unable to tune to it in any way. Recordings that were previously scheduled are still being recorded.
The channel does appear in the HDHR software and also in the Channels Web Client so it's clearly an issue with Android Client running on the Shield.
This happened once before to me and I think the only way I resolved it was to completely uninstall all my datasources and reinstall, but I don't remember for sure.