I saw that it is documented like that, but in my eyes that makes it pretty useless in a scenario where $du is filled by the path from registration of a device. That's why I thought it was a bug.
Do you have any suggestions to circumvent that problem? How would I save a per-branch variable? I tried something like `$avp(foo)[$T_branch_idx]` but it seems like Kamailio doesn't like variables being used for an index.