Install all updates, including any Surface firmware or Optional updates.
After the unexpected shutdown: Check Event Viewer-Look for:
Kernel-Power (Event ID 41)
BugCheck
Kernel-Boot
Surface ACPI events
====
Run a Power Troubleshooter
Open CMD as Admin and copy paste this command:
powercfg /batteryreport
Control Panel → Power Options → Choose what the power buttons do
Click Change settings that are currently unavailable
Uncheck Turn on fast startup
Save changes and restart.