From Meraki Support:
"Hi, Currently, the workaround is applied to the MX, therefore this is why you most likely haven't had issues with MX 18.210. Would you like to test by removing the backend workaround to see if the issue is present in mx 18.210?"
I do not know what the workaround was about but now clients are authenticated with 802.1X.
A little bit scare to remove it...