exactly same behavior on my side (tested also with ROS 7.2rc3)
I also opened a ticket at mikrotik - no solution till now.
It could also be the same topic as here: http://forum.mikrotik.com/t/hotspot-login-page-disconnect-after-while/154813/3
By the way - I disabled hotspot service (so it is a basic router) and everything works fine and stable - so it has to be an issue with the hotspot package.
It is not solved - please adjust topic!