Under Surveys, ask one person at a time how joining went, or why they're moving on. They get a Discord message with a link. This needs Nøyra Premium.
To protect anonymity, answers are kept together in groups of 5. Each group closes when it's full, and only then do its results appear under Insights. So answers can't be traced to one person by watching scores change, and it can take a while for results to appear.
Someone leaving needs to answer while they're still in your Discord server, so ask before they go.