What is the bug?

The tileBuilder function of a tile layer is only invoked on an image error if an errorImage has been passed. Therefore, it is not possible to construct a custom error widget in the tile builder itself.

How can we reproduce it?

Create a tile layer with a URL template which produces an error, e.g. https://example.com/{z}/{y}/{x}.

    FlutterMap(
      children: [
        TileLayer(
          urlTemplate: 'https://example.com/{z}/{y}/{x}',
          tileBuilder: (context, tileWidget, tile) {
            if (tile.loadError) { // This is never true
              return Icon(Icons.cancel);
            }
            return tileWidget;
          },
        ),
      ],
    );

Note that the builder never returns the icon since loadError is always false.

Do you have a potential solution?

In TileImage._onImageLoadError, notifyListeners() is only invoked in _display(), which in turn is only called if errorImage != null. To trigger a rebuild even if errorImage is not set, listeners should be notified in that case as well.