-tile-offset bug with tile:

[vallex]

Bad result , but faster (357ms):
convert -size 1024x1024 -alpha off -tile-offset -40-40 tile:Tex_0173.dds png32:tiles_bug.png
Good result, but slower (636ms):
convert -size 1024x1024 xc: -tile-offset +40+40 +size -tile Tex_0173.dds -draw "color 0,0 reset" png32:tiles_good.png

My question is : [-tile-offset with tile:] moves the whole image canvas or it's some kind of bug ?
Is it normal that doesn't fill the entire space of the image ?

Version: ImageMagick 7.0.5-4 Q16 x64 ; Windows 7

Here is the bad output:

..and the good one:

[fmw42]

Please supply your IM version and Platform.