Its actually the opposite, the shorter the channel width the more powerful the signal. Maybe you are confusing power and throughput? When choosing a channel width you also have to consider density, CCI and client types. Its important to note that Ruckus 802.11AC AP's will ALWAYS default to 80MHz channels. IME there is no point using the auto feature other than convenience of not changing as ive never seen an Ruckus AC AP choose anything other than 80MHz. There is a couple of reasons for this but the main one is throughput. Im positive the software makes the decision that because with 80MHz channels the throughput will be higher on average than 20 or 40MHz even if this causes CCI so it chooses that.
If you experience lots of client disconnects at 40MHz then there could be a number of issues for that, one could be a bug, did you try going down to 20? Another could be the channel that was chosen, was it the same centre frequency as when it was 80MHz?