There's now an option to download (Here) maps to use offline on the app start page - menu top right. Android only.
You only get the basic map which is better than nothing at least if you have features like roads and rivers.
It's a pretty big file beware if you havn't much space on phone.
When they used google maps they cached fairly reliably, in fact you could view an area on google maps and save it, the DJI App could access that saved version without previously vieing in the app.

Can only guess some kind of licencing issue is behind the change as they seem determined to tough out criticism.
To be fair I find the Here maps OK-ish and it's nice having something in most places
With google maps if you hadn't cached and had no signal there was literally nothing on screen.