Screen saver on remote desktop
After an afternoon of tinkering, a semi-solution.

I found I can fix the automatic launching of the screen saver when closing a screen share by shutting down and restarting the screen sharing service by issuing these commands in a terminal window:


  sudo launchctl unload
  /System/Library/LaunchDaemons/com.apple.screensharing.plist
  
  sudo launchctl load -w
  /System/Library/LaunchDaemons/com.apple.screensharing.plist


(See Restart Screen Sharing Service on Mac mini Server for more details)

This can be accomplished remotely by via SSH as well.

Another observation, the automatic launching of the screen saver only seems to "stick" once I open a screen share while a screen saver is running.  It's almost as if the machine "remembers" if you logged on while a screen saver is running and "decides" to start it back up when you logout... forever.
From then on the screen saver will launch every time you close a screen share session (until you restart the machine, or the service)

I hope this helps with watching Netflix. Sadly it doesn't help me much since I often launch a pretty screen saver on the TV just for aesthetics.  After that to go back to 'normal' I have relaunch the sharing service, which is highly annoying.  Here's hoping someone has a better solution...



Old answer:

Not an answer, but acknowledgement that someone else is having the same problem and some more specifics.  Hopefully someone can help.

Same setup:  TV connected to my Mac via HDMI.  I use screen sharing to control it.
More Info:  

1- The screen saver only launches when the last screen sharing client closes. (For example, if I'm using screen share on a laptop and iTeleport on my phone, if I close one or the other, the screen saver doesn't start up.  The screen saver will only start when the very last client disconnects.)

2- The screen saver launches even if the mouse is placed in a 'hot corner' set to 'Disable Screensaver'

3- If I reboot the machine, or disable and then re-enable screen sharing (using my connect mouse/keyboard) the problem goes away, until I run a screen saver once.  From that point on it will "auto-launch" the screen saver, until I reboot or disable/re-enable sharing.

4- I agree w/ OP that this problem must have come through an update or something because I have not noticed this behavior until just a few days ago after several years of using my mac mini with a TV connected as a monitor via HDMI.