@Teabone, I do have many of your signs and wall mount items. They do have the same issue. However, I cannot rubberband them or use a draw box when they are on a wall because the editor only discovers/recognizes the static wall model and so no draw box is available -- only the entity menu of the wall. I have attached 4 images. See the following descriptions.
This image contains a room (wall) and a dynamic door. I am pointing the cursor at the wall.
I am now pointing at the door.
I am now viewing the sign displaying how it is away from the wall a little too much. My cursor is not hovering over anything.
I am now hovering over the sign, the editor only discovers/recognizes the static wall model and so no draw box is available to use in order to select the sign so that I can move it closer. The bottom line, placement has to be spot on and all the parameters correct prior to placement. This might mean that you place the wall item outside of any room and not over any other entities, make the parameter adjustments. Such as the ifused or ifkey parameters.
Alienware Aurora R7 with SSD 256GB boot drive ( C: ) and a secondary drive ( D: ) that is 2TB
Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz 3.19 with Intel Turbo-burst
Installed RAM 16.0 GB
64-bit operating system, x64-based processor
Windows 10 Home
NVIDIA GeForce GTX 1070 with 8192 MB GDDR5 and 8095 MB shared system memory