Thanks for your feedback! Meanwhile I got several other tenants facing the same issue. Like I was saying device enrollment is working as expected but enrollment restrictions do not apply to user enrollment as expected.
Can't find any existing known issue on user voice, otherwise I had upvoted it already. Also, I don't agree this is a missing feature, it is rather a bug that is discovered as part as public preview of the user enrollment feature. I find this to be really annoying how to give feedback for bugs (not only for this MSFT product). A bug is different from a missing feature where you would up/downvote. A bug shall be fixed to provide the already defined feature set.
I have also opened a regular ticket for this, but it is also quite painful and extremely time consuming as this always feels like I as a customer (and on behalf of our joint customers) must proof that the bug is real rather than that MSFT is trying to proof that there is no bug. I mean, we're not in cort here but it always feels like we are and we are working against each other, not together. In the end, I am taking time, effort and money to help make Microsofts products better and don't feel this is really welcome by 1st and 2nd level support lines in Microsoft... they seem to be hired to block feedback away from the product groups. If I had no other ways to get in direct touch with the product groups, which definately not every customer has, I would be totally lost.