I've also found the offline use in Google Maps to be horribly unreliable. I had Google Maps set up with an offline map and, two times in a row, when I wanted to actually use it, the offline map had disappeared from my phone for whatever reason.

This was a good while ago, so I don't know, if it's still the case. All I know is that I switched to OsmAnd~, set that up once and since then always reliably had a map in my pocket.