It seems I have the same kind of behavior. I use RoboHelp
HTML (7.02.001).
I tried to add See Also keywords to topics, using the See
Also pane. A topic can be added to a See Also keyword. However,
when the topic is assigned to another See Also keyword, adding it
to a second See Also keyword failed without any error message.
An alternative is to add a topic to one or more See Also
Keywords in the Topic Properties dialog. I have three keywords
added to the topic, but the topic is only found in one of the See
Also Keywords in the See Also pane.
Later I found out I have to press the Apply button, before
pressing OK, when adding a See Also Keyword to a topic in the Topic
Properties dialog.
This seems as a bug in the See Also pane to me.
Kind regards,
Andre