I have noticed this recently, with my MalwareBytes Premium subscription.
It seems that Defender thinks that it is a full fledged antivirus product and says
Which I suppose is sort of true.
I had just grown used to them playing better together, along side one another.
Control Panel\All Control Panel Items\Security and Maintenance\Virus protection
Says.....
"Malwarebytes is turned on"
Which of course it is, but I have never considered MalwareBytes a complete, comprehensive and robust "antivirus" solution.