Hello @spyromus ,
I experienced this one before, here my insights ;
Documentation says :
The editor will automatically generate a convex shape around each tile, taking into account pixels that are not 100% transparent:
Here is what is Convex wiki.
A post about this situation.
Here is how engine treated 2 different shapes with using shapes alpha.
** Concave Left , Convex right
Best Regards,
PS : if there is another info that i didn’t see yet, feel free to flag this post.