Border colour around reprojected GeoTIFF is not rendered as transparent one

A GeoTIFF image reprojected from one CRS to another is rendered as it has value of "0" outside of original image boundary:

Steps to reproduce:

1. Define custom CRS in Geoserver:

   100101=PROJCS["UM PL",GEOGCS["UM PL GeogCS", DATUM["World Geodetic System 1984",SPHEROID["WGS 84",6378137.0,298.257223563]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9108"]],AXIS["Longitude", EAST],AXIS["Latitude", NORTH]],PROJECTION["General_Oblique"],PARAMETER["central_meridian", 19.3],PARAMETER["latitude_of_origin", 37.5],PARAMETER["scale_factor", 1.0],PARAMETER["false_easting", 0.0],PARAMETER["false_northing", 0.0], UNIT["degree",1.0,AUTHORITY["EPSG","9108"]]]
   

2. Define simple style:

   <?xml version="1.0" encoding="ISO-8859-1"?>
   <StyledLayerDescriptor version="1.0.0" xmlns="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc"
     xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://www.opengis.net/sld http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd">
     <NamedLayer>
       <Name>gtopo</Name>
       <UserStyle>
         <Name>dem</Name>
         <Title>Simple DEM style</Title>
         <FeatureTypeStyle>
           <Rule>
             <RasterSymbolizer>
               <Opacity>1.0</Opacity>
               <ColorMap>
                 <ColorMapEntry color="#000000" quantity="-1000" opacity="0"/>
                 <ColorMapEntry color="#AA0000" quantity="0"/>
                 <ColorMapEntry color="#00FF00" quantity="10"/>
               </ColorMap>
             </RasterSymbolizer>
           </Rule>
         </FeatureTypeStyle>
       </UserStyle>
     </NamedLayer>
   </StyledLayerDescriptor>
   

3. Import attached GeoTIFF file (UM1_OROGRAPHY.tiff) and force CRS to above mentioned
4. Display image as EPSG:3857 or EPSG:4326