I'm using a Toshiba Satellite. I've checked the power settings, and when plugged in the machine is set to never suspend. I didn't have this problem on Loki, so I'm assuming it's a setting I'm not seeing in the Settings, or can be solved by command line.
Do you have tlp (linux advanced power management) installed? If the answer is yes you could install the tlpui and disable the power saving mode when in battery, and see if it still happens.
To install tlpui :
sudo add-apt-repository ppa:linuxuprising/apps sudo apt update sudo apt install tlpui