QUESTION:
Can telegrams partially be routed faulty with intact bus coupler?

ANSWER:
If three or more busses are connected via bus coupler units, and on (at least) one bus coupler unit the station addresses of both bus coupler modules are set equal, the following behaviour may happen:

Telegrams running through (at least) two bus coupler units are routed faulty or don't reach their target destination.
Telegrams between two neighbouring busses are not affected.

Remedy:
Both station adresses of the bus coupler modules belonging to one bus coupler unit should be set unequal.