This problem concerns WMS, group layers and, apparently, images with transparency. I start by creating a group layer containing two layers. Each layer sources its data from a GeoTIFF image and the images have RGBA bands. I load the group layer in a client via WMS. I find that there is a range of requests which fail to draw. I have included some example requests below. The logged error is included below too.
If I replace the source GeoTIFFs with RGB (no transparency) the problem does not occur.
If I add the individual layers (ie the layers that are included in the group layer) separately to the client, the problem does not occur. But if I add the individual layers to the client in a single request (ie layers=layer-1,layer-2) the problem does occur.
I've uploaded the information that is logged when draws fail in
I've uploaded some sample data in
which I'll describe in a comment.
I have set the affects version to 2.15.2. I have tested the issue in previous versions of GeoServer and found that it does not occur in 2.14.0, but it does occur in 2.14.1 onward.
Oracle Corporation: 1.8.0_162 (Java HotSpot(TM) 64-Bit Server VM
Native JAI: false
Native JAI ImageIO: false