When selecting the Full data migration option, vSAN will try to evacuate all of the components that are currently hosted on that host. If any objects have a storage policy that will not be compliant after the host enters maintenance mode, the action will fail, as vSAN will always enforce the objects to be compliant to the policy (except when specified otherwise).
For Example: If you have 3 hosts and an object with failures to tolerate of 1, the minimum requirement to be compliant with this policy is 3 hosts. If you try to evacuate one host with maintenance mode, this will leave you with 2 active hosts, which is not enough to comply with the specified policy.
This rule also applies to the objects that are not currently registered in the inventory. For Example: If you have a virtual machine with a certain storage policy and you remove it from inventory, it will still be a known object on the vSAN datastore, which means vSAN still needs to keep it compliant. The same goes for the virtual disks for example that have been detached from the virtual machine.