Macbook Pro won't display on 2nd external monitor until restart
I'm having success so far at least by trying this answer to a similar question posted in 2012. I wasn't looking back that many years because I thought it wouldn't apply, but it looks like the same problem. 

Switching my TV to Thunderbolt Port 1 on the Mac seems to be avoiding the problem for the moment. Now using Port 2 for my other external monitor, but since that's a regular computer monitor, perhaps that avoids the same problem happening with it.