It's is a known issue for use here: https://youtrack.jetbrains.com/issue/RSRP-450266
This bug is going to be fixed in the upcoming ReSharper 10.0.1 update.
As a workaround until the hot fix is available, avoid clicking on parent nodes in the tree. If you only click on tests, the output pane works as expected. Once you click on a parent node, the output pane gets "stuck", and stops updating. If this happens, right click on the tests you're interested in and select "Create new session". A new tab will open and you'll be able to run the tests again.