explicit -d uint save result: 
uint8.tif            :  128 x  128, 3 channel, uint8 tiff
    tile size: 16 x 16

unmodified copy result: 
tmp.tif              :  128 x  128, 3 channel, uint8 tiff
    tile size: 16 x 16

copy with explicit -d uint16 result: 
copy_uint16.tif      :  128 x  128, 3 channel, uint16 tiff
    tile size: 16 x 16

siappend result: 
tmp.tif              :  128 x  128, 3 channel, uint8 tiff
    tile size: 16 x 16
 subimage  1:  128 x  128, 3 channel, uint16 tiff
    tile size: 16 x 16

combining images result: 
tmp.tif              :  128 x  128, 3 channel, uint8 tiff
    tile size: 16 x 16