does windows go through the shutdown process? could be windows is closing but the bios isn't shutting pc off, it can happen if there is a conflict.
is this windows 10? the way it works by default, a restart is the only time PC is actually powered down, whereas on a shutdown its going into a hybred hibernate. So its odd it happens on a restart only
never seen a pc go to sleep instead of restart, that is a strange reaction
if you are on win 10, try this
right click start button
choose powershell (admin)
type SFC /scannow and press enter
once its completed, copy/paste this command into same window:
Repair-WindowsImage -Online -RestoreHealth
and press enter
SFC fixes system files, second command cleans image files, re run SFC if it failed to fix all files and restart PC
what are specs of the PC? Could be we just need to update some drivers to get it to work properly.