I use radius, just not for dhcp. I can tho. Do you have radius logging enabled?
/system logging
add topics=radius action=memory
For dhcp, User manager should only return Access-Accept and the ip address (Framed-IP-Address for FreeRADIUS users) or the ip pool (Framed-Pool) when the mac address is sent. You can only return attributes that apply to dhcp. No Mikrotik-Group or anything like that.