I'm running BTT 3.209 (1386) on Mojave (10.14.6) on a MacBook Pro (15-inch, 2018).
In the Clipboard Manager window, if one item in the list on the left is highlighted and I double-click on a different item to insert it, sometimes the item that is highlighted (i.e. the wrong item) gets inserted instead. In this screenshot, Clip 1 is double-clicked, but Clip 2 is inserted, because it is highlighted.
In order to be sure that the correct item gets inserted, I have to single-click on the item I want, wait a short time (so it doesn't register as a double-click), and then double-click.
This doesn't always happen. Sometimes the correct item is inserted even if another item is highlighted. The wrong item gets inserted most often if the clipboard manager window was not active (but was still floating on top) before double-clicking, but it also happens occasionally when the window was active to start with.
I've been using BTT for a while and it's always been like this.