For what it's worth, I'm currently reading Antony Beever's Ardennes which is extraordinary. I strongly recommend it.
Your Grandpa experienced some really serious stuff and I'm profoundly glad to hear he made it through.
During the Ardennes Offensive, areas liberated by the allies in Belgium were then reoccupied by German forces, leading to many arbitrary executions of civilians seen as collaborators.
Just finished reading Ardennes 1944: Hitler's Last Gamble which details these events, very harrowing.
Yes, this did happen and retribution was carried out by certain German units.
I read about it recently in Anthony Beevor's 'Ardennes 1944: Hitler's Last Gamble'.