I’ve been using an HDHomeRun Flex 4k tuner for 1 week (sent by Silicondust as warranty replacement of my failing HDHomeRun Flex Quatro) and it was working great with the Channels Apple TV beta app. All the network affiliates in the Washington DC area are broadcasting in ATSC3, and I switched my Channel Collections to use the 3.0 channels instead of the ATSC1 channels.
My HDHomeRUn Flex 4k had then-current 20221205 firmware, but a couple of days ago I updated its firmware to the new 20230303 version. I immediately noticed that with the new firmware it takes a very long time to tune in ATSC3 channels on the Channels Apple TV beta app, as much as 20 seconds every time. Once it starts it plays fine. With the previous Flex 4k firmware, the Apple TV beta app (which I have been using all along) was able to tune in ATSC3 channels just as quickly as ATSC1 channels, in less than 2 seconds.
At first I thought the issue was a problem with the HDHomeRun Flex 4k 20230303 firmware upgrade (and I was going to try and downgrade it) but the HDHomeRun Windows and iOS apps still do not have any delay in tuning ATSC3 channels, its occurring only with the Channels Apple TV app (on 2 separate ATVs).