Thanks for the detailed description of the problem.
I just finished debugging, and this is a bug in Online Maps.
The bug is fixed, and the new version will be available through the built-in update system in 24 hours.

It seems I have a similar problem as the last post on this forum.
I have downloaded tiles using the TileDownloader.cs script from this post, and placed them inside a OnlineMapsTiles folder located in Assets/Resources.

I downloaded zoom levels 12 to 16 and I never go under 12 or above 16 when testing.
I cleared the cache to simulate a "fresh installation" of the application on a new device.

Unfortunately the tiles are not always displaying correctly.

In the Editor:
- Count Parent Levels at 5 -> the map starts grey and the tiles show up correctly only if I use zoom level 14 or above. Zoom levels 12 and 13 are not loading and the maps stays grey.

- Count Parent Levels at 0 -> Tiles load correctly except for zoom level 12 (grey), but we can see the tiles loading in real-time (from left to right) and I'd like to avoid that.

In standalone build:
- Count Parent Levels at 5 -> Approximately the same as in the Editor.

- Count Parent Levels at 0 -> Almost nothing loads.

My Map source is set to Resources.
Resources Path is set to : "OnlineMapsTiles/{zoom}/{x}/{y}"
Zoom is clamped between 12 and 16.

What am I doing wrong here ?
Thank you !