This sounds like you might have one of the Sagemcom hubs, if you have you will need to request Wi-Fi Optimisation to be disabled.
However, the way that WiFi interference is dealt with in the 5GHz is different to the 2.4Ghz band. It will never give you exclusive use of which channel you set in each group (36 - 48). Take the one you mention, whichever channel you set it to in that group, the router will try & use at first. However, if it detects other routers using (say 40 in your example), then it will try & use another channel in that same group & you might find it on perhaps 48. Then if that channel is congested, it might move to 36. Then channel bandwidth needs to be considered, if someone is transmitting enough data to warrant 80MHz bandwidth, that router will try & use all four channels. All routers will try and manage the bandwidth to give everyone some degree of service. You cannot change this behaviour, as it is the way every router or AP works in the 5GHz band.
