Some parameter change were introduced in 1.5.5_0181 to adjust WiFi power saving mode. This issue could be one of the side effects (not detected in our lab test environment, could be compatibility issue)... A new version to rollback the change is in beta now. Available once your MI ID is in the white list.
BTW, we also fixed some WiFi defects which may cause the device to go offline. The official release will be ready in a couple of weeks. Please stay tuned.