Late to the party here, but had to join the forum just to show support on this one issue. (Wax Bean) you're right - I can't find any other mention on this subject in the universe other than 1,000 generic "fix google maps!" ad-laden sites. I've had exactly the same issue and simply can't believe no one else has... as I've had it now for probably two years across four devices of various brands and revisions. My guess is there just aren't that many folks not contracted into an unlimited data plan. I use pre-paid data on all my devices that have mobile data, and leave it off unless necessary. I then see this issue all the time; Google offline maps works great one day, then suddenly starts behaving like it's working with only downloaded cache. If I've been to a full-zoom/resolution "block" recently, I'll have full detail for that block of the map, but other blocks I haven't visited recently are in "base map" mode with ultra-low resolution (triangular lakes, only interstates or state routes showing, and no points of interest).
What makes it odd, and is key to this whole weird condition, is that the data exists - Maps knows where all those POIs are, what their hours and phone numbers are, it just won't display them unless searched on individually. And then it puts the pin down on an otherwise blank area. "Your store is here, open till 10pm, here's their phone number... but they're invisible...." Same with roads. It knows they're there, but won't display them. To Wax Bean's point; I can navigate down those roads, they just don't appear on the map. If I didn't have the map, navigation would have failed with "no route exists", but that's not what happens. The blue line shows up, going down invisible roads, and voice knows "turn right, turn left" during navigation, and all the road names are listed on the textual "steps" page, I'm just on an otherwise empty screen. The full map exists, and Google knows it's there - it just chooses not to display it. One tap of "mobile data" and bam - it all comes flooding back. And faster than it could download. I don't appear to be using enough data to re-download any portion of the map, it's just like Maps is mad that it hasn't been tickled lately. My best interpretation or analogy is that navigation is running off of the offline map, but the graphics are using separate cache data. Turning on mobile data (or wifi) somehow wakes it back up and it then backfills the cache with offline maps. Which lasts until the next time it purges some old cache then goes offline.
As for coincidences or influencing factors; three of my devices I DO have maps loaded on external SD card, but the fourth device experiencing the problem only has internal memory. I would entertain the SD-card-related theory if it weren't for the fact that, again, the data is there - it's just not displaying on the map. I also have multiple versions of Maps running on each device, from Maps 9.x.x.x to 10.77.0. One device is an old Nexus, one a recent Samsung, one an older Motorola, and a recent Motorola. All have the issue from time to time. Restarting Maps and/or the device does nothing. The only "fix" is either re-enabling data and then viewing the desired "blocks". Alternatively removing and re-downloading an offline map "fixes" the entire map. "Removing Google Maps" (uninstalling updates) and reinstalling only works in so much as it re-downloads the offline map.
The only coincidence remaining in my head is that I didn't notice this ill behavior until about the time Google extended their offline maps expiration from a month to a year. Unknown if the fresher maps didn't have time to begin experiencing (what I'm calling "the cache") problem, but my current "workaround" is to simply make sure and update my offline maps every week or so, whether they need it or not. That seems to keep things fresh and avoid the issue.
Not suggesting this as a solution - I'd love to hear what the "real" answer is. But I maintain that it would probably happen to any device, it's just that Wax Bean and myself may be the only people not daily connected to data.