When a superior force joins another force, the message states that the superior force joines itself
I tested it in the Finland scenario. I moved the Army of Finland to merge with Haapamaki division, and the message that appears is:
"Army of Finland joined Army of Finland"
(it should read Army of Finland joins Haapamaki Division, or vice-versa)
This is probably an Age-engine problem, as it is not RUS specific.
See pic
(No save attached, as it is not needed, it suffices to launch the Finn scenario and move the Army of Finland to merge with the division)