The drop down list has 20, 40, and 80. I don't see 20/40 or 20/40/80. Does selecting 40 automatically give you 20/40 or does selecting 80 give you 20/40/80?
I'd guess it would depend on the actual device, but some information about values available here: https://wiki.openwrt.org/doc/uci/wireless#mac80211_device_options. You can check the
/etc/config/wireless file after setting values with Luci.
hostapd automatically downgrades. You are not setiing the channels bandwith so much as you are setting the MAXIMUM it will go up to.