gdalwarp -dstalpha doesn't tag the last channel as alpha
|tiff alpha gdalwarp
If doing "gdalwarp in.tif out.tif -dstalpha" where in.tif has 4 or more bands, out.tif will not have its last added channel tagged as being alpha in the TIFF file. It will be only written in the .aux.xml file.
Furthermore, if adding -co ALPHA=YES, the second band will be tagged (in the TIFF) as alpha. But this latter behaviour is consistant with the documentation of the ALPHA creatoin option : "The first "extrasample" is marked as being alpha if there are any extra samples"