Some subchannels and SD main channels try to tune, but the spinner hangs and it kicks back to the main Channels screen. It looks like difficulty with many 720x480 streams. The channels work with the hardware decoder, but that eventually causes the app to hang and the Fire TV has to be restarted (this is not new). Possibly tied to the libraries update or stretch to fit setting added. I tried disabling the stretch with no change. Happening on multiple sticks.
Fire TV Stick 4K
Channels for Fire TV 21.1.11
Fire TV OS 184.108.40.206(ns6265/2157)
Fire TV Home Version 220.127.116.11-713