on the new Fire TV Stick 4K Max. I know this is a new device and there may be some bugs to work out. I am seeing some jittery motion on the ATSC 3.0 channels whether live or recorded on the Channels DVR app while the native HDhomerun app looks OK. the tuner is HDHomerun Flex 4K
Can you check what setting is under Settings > Playback > Advanced > Video Decoder
it's on Hardware.