Basically, there is a requirement of receiving device-public-mac attribute in order to check the device compliance against MDM based on the MAC address, but even though some of the PCs are sending this attributes, mobile devices are not. It was tested on Android v10 with the latest v4.9.00548 AnyConnect.
Based on the documentation this is possible via ACIDEX attributes, but what are the conditions for sending ACIDEX attributes from AnyConnect client? So far, it looks like platform dependent, but I haven't found any reasonable documentation and there's no definite answer from some of the internal sources as well.
AnyConnect Identity Extensions from Mobile Devices
• AnyConnect Mobile (iOS, Android) do not send MAC address – not available through OS API
• Currently ISE uses MAC address as key for Profiling, MDM lookup