So after further investigation, it turns out any time the dev server reloads (any code change), every workflow instance disappears. This is really annoying. Is this expected?
You can repro with the workflows starter template easily.
Just run it, go to localhost:8787, record the instanceId and note that localhost:8787?instanceId=<id> shows details of the instance, add a console.log call anywhere and save, now localhost:8787?instanceId=<id> gives