Err... ok, I think I understood...
I mixed up the shortkeys (in my mind) and was suppressing tiles from canvas instead... so there were no tiles in the tiles panel tiles... And I didn't look at it until now x)
But still, PyxelEdit was exporting an empty wide image instead of an empty one matching a base tile dimensions :p
May a dialog box warning us of empty tileset could be cool, isn't it ? :D