Not sure if it is bug or i just don't understand something.
Tile Stamps works fine when i use "Stamp Brush" tool – it randomly places all the variants.
But when i try to use tile stamps with object layer "Insert Tile" tool, It switches between these tile stamps, but places only one image from variants list and not use these variants.
P.S. Just realised that random mode doesn't work with "Insert Tile" tool either.