Update: I have tried this five times now and its worked all five times since this initial issue. I think it was happenstance and that due to a weak signal the call dropped at the same time.
I now consider this a non-issue.
I was on a call and realized bluetooth was off. So I enabled it via General settings, but the panel for the call did not change from “Speakerphone” to “audio sources.” I pressed the button on my Jabra JX-10 and the sound transferred and then the call hung up.