MapTileProviderArray with two DownloadProviders / combining regional & global online maps not working

[secaw]

Issue Type

[X] Question

Description and/or steps/code to reproduce the problem

I want to combine two Online Map sources, one is i.e. NOAA nautical charts for the US costs only, the other (background map) is OpenStreetMap. I have a custom MapTileProviderArray running which is capable of overlaying multiple offline maps with a background online map for example. But with two online maps this is not working, because empty tiles (I would guess HTTP 404) of the regional foreground map are registered as found and completed, thus the array looking no further in the provider chain and the empty tiles being displayed in light grey. I've searched up and down in the osmdroid source code, but could not find, where the empty tiles are being rendered. Any help is very much appreciated!

PS: I know I can load the NOAA map in a map overlay, which is working nicely in transparent mode, but then for each requested tile two tiles will always be loaded, although only one will be displayed, so this unfortunately is not a solution.