According to Firearm Chronicles
Last week, Michigan FFLs received a letter from the federal Bureau of Alcohol, Tobacco, and Firearms (ATF) notifying them that a Michigan CPL will no longer be exempt from the NICS check when purchasing a firearm. This applies specifically to FFL dealers, not to private party sales. There is also no impact on RI-10 purchase permits.
While CPL holders are used to this simplicity in purchasing firearms, it has only been allowed since 2006. Many Michigan gun shops ran the NICS check anyway, despite the CPL, during this time. The ATF has recommended the NICS check as a best practice to reduce liability exposure.
The ATF letter is vague in a lot of respects. It references that the agency received information from the FBI that Michigan CPLs continue to be issued to people prohibited under federal law from possessing firearms including those convicted of misdemeanor domestic violence and those using marijuana. There is no other specific reasoning given. There are theories that the ATF’s change in position is because MSP hasn’t been doing NICS checks on CPL applicants but there is no evidence supporting that.