Google's Android OS used to have an annoying feature - smart network switch - which would inevitably lead to it sitting there, not using your wireless network, displaying the message "No network access".
This usually happened as you got home, because it had picked up your wifi at the very extreme edge of it's reach, and the test probes had failed as a result.
The functionality works by placing some test HTTP requests when connected to a wifi network - if those requests fail, it's considered that the wifi doesn't have network access. This (fairly flawed) methodology doesn't properly account for a range of possible failures in the test itself.
Unfortunately, Xiaomi appear to have felt the need to replicate this behaviour in MIUI - the only real difference is that Xiaomi's functionality displays "Can't Connect to Internet" on the wifi network.
The result is that having previously been to the margins of your coverage, you'll eventually notice that your phone is relying on mobile data instead of wifi, and has eaten your battery (and your data allowance) as a result.
Anecdotally, the issue seems to have become more prevalent recently.