Yeap, according to my experience they do rescan from time to time and maybe even roam once in an hour. Indeed at the beginning the AP is chosen based on the signal strength.
I suggest you to reduce transmit signal strength on the farthest AP below the nearest APs signal (-55dBm is below -50dBm). Strong remote signal confuses devices. For this task use a laptop with some free WiFi Scanner software, which displays actual signal strength values in the room.
Alternatively use Force BSSID in HAA Setup and forget.