I Hadn't update to 8 from 7 for months and wait until 8.1.2 'cause I knew there'll be issues for a new release.
but no luck, this issue came up.
well, you don't have to restart every time the issue occurs. You can just go to Setting and turn off and back on of Multitasking Gestures feature.
but it is really a irritation when I bump to this issue.
i Want to request those development dep. bench runs very carefully before releasing any updates.
this is not very professional work and would lose tremendous number of customers including myself.
in fact, i kind of gave up IOS and bought non IOS device for my first smart phone.
good luck IOS loves!