Failure to transform vertical unit between 2 compound CRSs with boundCRS on horizontal and vertical parts

[rouault]

projinfo -s -t with the following input CRS

COMPD_CS["NAD83 / Alabama West + NAVD88 height - Geoid12B (Meters)",
    PROJCS["NAD83 / Alabama West",
        GEOGCS["NAD83",
            DATUM["North_American_Datum_1983",
                SPHEROID["GRS 1980",6378137,298.257222101,
                    AUTHORITY["EPSG","7019"]],
                TOWGS84[0,0,0,0,0,0,0],
                AUTHORITY["EPSG","6269"]],
            PRIMEM["Greenwich",0,
                AUTHORITY["EPSG","8901"]],
            UNIT["degree",0.0174532925199433,
                AUTHORITY["EPSG","9122"]],
            AUTHORITY["EPSG","4269"]],
        PROJECTION["Transverse_Mercator"],
        PARAMETER["latitude_of_origin",30],
        PARAMETER["central_meridian",-87.5],
        PARAMETER["scale_factor",0.999933333],
        PARAMETER["false_easting",600000],
        PARAMETER["false_northing",0],
        UNIT["metre",1,
            AUTHORITY["EPSG","9001"]],
        AXIS["X",EAST],
        AXIS["Y",NORTH],
        AUTHORITY["EPSG","26930"]],
    VERT_CS["NAVD88 height",
        VERT_DATUM["North American Vertical Datum 1988",2005,
            EXTENSION["PROJ4_GRIDS","g2012a_alaska.gtx,g2012a_hawaii.gtx,g2012a_conus.gtx"],
            AUTHORITY["EPSG","5103"]],
        UNIT["metre",1,
            AUTHORITY["EPSG","9001"]],
        AXIS["Gravity-related height",UP],
        AUTHORITY["EPSG","5703"]]]

and output crs

COMPD_CS["NAD_1983_StatePlane_Alabama_West_FIPS_0102_Feet + NAVD88 height - Geoid12B (US Feet)",
    PROJCS["NAD_1983_StatePlane_Alabama_West_FIPS_0102_Feet",
        GEOGCS["NAD83",
            DATUM["North_American_Datum_1983",
                SPHEROID["GRS 1980",6378137,298.257222101,
                    AUTHORITY["EPSG","7019"]],
                TOWGS84[0,0,0,0,0,0,0],
                AUTHORITY["EPSG","6269"]],
            PRIMEM["Greenwich",0],
            UNIT["Degree",0.0174532925199433]],
        PROJECTION["Transverse_Mercator"],
        PARAMETER["latitude_of_origin",30],
        PARAMETER["central_meridian",-87.5],
        PARAMETER["scale_factor",0.999933333333333],
        PARAMETER["false_easting",1968500],
        PARAMETER["false_northing",0],
        UNIT["US survey foot",0.304800609601219,
            AUTHORITY["EPSG","9003"]],
        AXIS["Easting",EAST],
        AXIS["Northing",NORTH],
        AUTHORITY["ESRI","102630"]],
    VERT_CS["NAVD88 height (ftUS)",
        VERT_DATUM["North American Vertical Datum 1988",2005,
            EXTENSION["PROJ4_GRIDS","g2012a_alaska.gtx,g2012a_hawaii.gtx,g2012a_conus.gtx"],
            AUTHORITY["EPSG","5103"]],
        UNIT["US survey foot",0.304800609601219,
            AUTHORITY["EPSG","9003"]],
        AXIS["Gravity-related height",UP],
        AUTHORITY["EPSG","6360"]]]

fails to to do the only required change: a vertical unit one from metres to feet