When using the advanced projection handling to reproject from EPSG:3035 to EPSG:3857 or EPSG:4326, vertical gaps (nodata) in the image can be produced.
The gaps are not always produced, as it seems to depend on specific combinations of:
To reproduce with 2.15.1:
Download the attached test.tif file
Publish it as a standard GeoTiff store, with the standard "raster" style
Ensure WMS Advanced projection handling is ENABLED
See the gap with the following WMS request (adjust host, port, workspace and layer name to your environment)
You can see the result (with the gap) in the attached file cite-test.png
I'm attaching 2 other examples of different areas with different styles.