I was interested in your observation because I never attempted this, and I tried to replicate your problem.
I opened a linux guest and suspended the host (Windows 7 x64, WS 15.5.6) manually by Start -> Shut Down -> Sleep.
When I woke the host up, I found the guest exactly as I had left it, logged in and all applications still running.
So maybe this is a Windows 10 thing? Or maybe you use some special features (e.g. guest snapshots, I don't know) and Workstation in some cases is programmed to logoff to protect the integrity of the guest? I'm sure some more advanced users have more ideas.
Also, as it may be relevant for troubleshooting, how much RAM does your host have and how much have you provisioned for the guest?