When creating a series of models I tried to optimise the number of used files.

I created one mtl-file and several obj-files.
the structure of the files is:
doors.mtl
Door1 (subfolder)
Door1.obj
Door2 (subfolder)
Doors2.obj

Each Door*.obj file is referencing the mtl-file with mtllib ../Doors.mtl .

I created a furniture library.
Placing the door in the plan the door consists of all material in the right colour and texture. Even modifying colours for some parts is working.

But when saving the plan the mtl-file got lost. It will not be located directly in the root folder where I expected it to be.

When loading the plan again all material is in default colour and any parts are not displayed in the Modify materials dialogue.