Previously downloaded cities disappear from the application for Android

The downloaded city may disappear due to one of the following reasons.

2GIS data can be deleted by third-party applications that clean a device memory — Android Assistant, for example. Disable these apps and check whether the problem keeps occurring.
On a few devices, 2GIS periodically loses access to the SD card, and that causes the loss of the city data. Go to the side menu → “Settings” → “Data location”. If you’ve previously chosen to keep data on the external memory, switch to the internal storage instead.