Tim,
Thanks for chiming in, but I don’t think this is it. The bug depends on the piece under it being invisible. If the piece under it is visible, then it stacks perfectly on top of it, but if the piece is invisible, then it is displaced by half a hex (both up and to the side). Note this is not an “expanded” stack issue. The spacing on expanded stacks is much smaller. When the unit underneath is visible, the two units are clearly stacked.
If the unit underneath starts invisible and then is made visible, the top piece will “snap” back into the centre of the square.
That’s what made me think that there must be some code somewhere that says under certain circumstances (like these two pieces can’t occupy the same hex) then if a player tries to move a piece into that hex, it will move it partway in until the conflict is resolved.
There’s no mechanic like that that you know of?
Ken
P.S. More details are available in the bug report