In my Win 10 I have three Oracle clients.
I did connect to the database (three different schemas) with "Connect Using: client2"
After some time I did "Test All connections (Reconnect)" and Toad reconnect using client1 instead of previously selected client2.
BTW I need to reconnect after some time of not using Toad as my databases/network are configured to disconnect/drop connection.
I saw this in Toad beta 18.104.22.1686 but don't know whether this problem exists in other versions.