Hi,
This is a problem that I've been having for quite a while. It has been consistantly happening in El Capitan with Teamviewer 11 and now with Sierra using Teamviewer 12. When I updated to v12 it didn't do it the first couple of times that I tested it but has since done it every time. My screen ius set to turn off after a minute of inactivity and it does except when my iMac is accessed via Teamviewer. I use it as a media server so I don't want the computer to sleep but I do want the screen to turn off.
Thanks,
SXBB