Try moving the mouse into the Collections panel, hovering over the newly selected collection, and then hovering over the previously selected source. Also, see if the problem occurs in a new catalog.
I have observed in the past that LR sometimes doesn't update changes to the selected source made both by the user and by plugins; this problem occurred on some but not all of my catalogs. See this bug report: Lightroom 5.7.1: Highlighting of selected sources doesn't always update correctly
Irritatingly, or not, I tried it this morning, no changes to the plugin from last night, and the collection panel updates correctly. I hate these intermittent things, don't you.
I've never, I don't think, had it happen interactively; this is the first time I've seen it.
It seems to be intermittent, sometimes works sometimes doesn't. I'll post back if I can nail circumstances.
Try moving the mouse into the Collections panel, hovering over the newly selected collection, and then hovering over the previously selected source. Does the display then update properly? If it doesn't, it's likely the same issue I observed. If it doesn't, it may be a different issue.