@Michal Koch, Thanks for posting in Q&A.
From your description, I know you cannot enroll your new Xiaomi MIX flip to Intune.
Based on the official document, Xiaomi HyperOS is not in the supported operating system list, so that is the reason why you cannot enroll your device in Intune.
If you want Xiaomi HyperOS to be in the operating system list, you can submit your feedback to Intune.
https://feedbackportal.microsoft.com/feedback/forum/ef1d6d38-fd1b-ec11-b6e7-0022481f8472
Thanks for your time and understanding.
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