Didn't really got what do you mean by tilemap sprite, normal tilemap does not have a configurable colission box as far as i can see.
First of all you can try to change jump-thru to solid and check if anything changes.
Also you can try to disable platform behaiviour by default and make the following : on start of layout -> wait 1.0 sec -> enable platform behaiviour, and see if anything changes.
As a work-around, you can use normal sprites with a jump-thru behaiviour with 0% opacity and place them under the platform image.