It look like the same bug here: http://www.miui.com/thread-2929706-1-2.html
I use my laptop to be a hotspot, test for a whole night, no any wifi issue... so it's compatible issue with some wifi router.
Have a similar issue in France with the Freebox (internet box of Free operator) : the phone connect to wifi but internet doesn't work. After a reboot internet seems to work for a while but we have to systematically reboot the phone