TMX in WorldServer Package are not fully imported in sdltm

Hello,
we receive from one of our customers worldserver packages where many TUs from the TMX files are not included in the corresponding sdltm files, that Studio creates in the background.
I made some analyzes and found a common property of these TUs: they have a prop element:
<prop type="x-StructureContext:SingleString">restype</prop>

Why are such TUs not imported in the sdltm files? Is it a bug in Studio or does this come from a custom configuration in WorldServer?

Thank you
Sébastien Desautel