Possible to be something from tvheadend server 4.3 and new deinterlacing method?
Maybe try to install the tvheadend server 4.2. No need to unistall the 4.3, just disable. Search only one transponder for channels where the issues appear and check.
I have 3 RPi3B in network, one I upgraded to LE 10.0.2 few months ago, without to experience any unusual issues with LiveTV.
Recently I upgraded the last two. Upgraded the tvheadend server to 4.3 (with 4.2 had some EPG updating issues, what is present in 4.3 too) and very seldom, was issues to start the LiveTV (only 1080i, stream blocked at first picture, in the media info "no deinterlace", at all clients, not only where the server running), two days ago I returned to 4.2 version, no issues yet. The "memory bandwidth required for deinterlacing" could be an explanation, in my case when the stream starting use too much memory/processor and lead (?) to deinterlacing issue... maybe... (RPi3B - smaller uP).