Someone on another forum gave me the solution:
do you use AV? because it sure sounds like your AV hasn't updated the registry
https://support.microsoft.com/en-us/help/4088876
that's the latest CU, but it's been that way since Jan (Meltdown / Spectre vulnerabilities)
I added the regkey on the problem clients and after a reboot I got the updates of January, February and March.