This problem seems related to Bridged wifi ap, DHCP Offers only reaching clients after 5 min period and Disappearing DHCP Offers & bridge ageing_time. After setting the Ageing time of all bridge devices in LuCI on all network hardware to 10 instead of the default 30, the problem is less noticeable (but still there). Devices roam over and get an IP address within a couple seconds, and sometimes the transition is seamless. So whist this is not solved, there is a kind of work around. Still looking for a more permanent solution.