Please note this “preferred band” option gives priority but not exclusivity to the selected band, meaning that this is how it is supposed to work. What may be happening here is that the wireless adapter is not connecting to 5GHz band all the time for a variety of reasons and then it chooses 2.4 GHz instead. If that is the case you may want to find out why it does not connect to 5GHz always.
Another possible workaround is to disable your 2.4GHz profile from connecting automatically or add it to the exclusions list in Windows* wireless zero configuration or Intel® Proset Wireless Connection Utility. In that way you may also connect to other 2.4GHz networks in different locations.