TestFlight: Moving internal tester to new group removes them from test program

I am changing a group of internal testers from automatic to manual distribution of an iOS app. I need to do this seamlessly to avoid disrupting ongoing testing.

A group's distribution method (automatic vs. manual) can only be set when creating a group, so I created a new group set to manual distribution and moved users to the new group like this but it doesn't work as expected:

At start: Internal testers are in an existing group set to automatic build distribution.

1. Create new group set to manual build distribution.
2. Add new group to same builds as existing group.
3. Add tester to new group.
4. Delete tester from existing group.

I expected the tester to be able to continue testing as a member of the new group. Instead, the user's TestFlight says "Tester Removed" and "The developer removed you from the test program". But the TestFlight admin web page shows a green checkmark and "Installed" next to the user in the new group. Status in the tester's TestFlight app doesn't match the TestFlight admin page.

Is this a bug in TestFlight, or did I miss a step?

Waiting 30 minutes for TestFlight changes to propagate doesn't help. Having the user uninstall and reinstall TestFlight doesn't help. TestFlight still says "Tester Removed". The only solution seems to be completely removing the tester from the app and adding them back in the new group (swap steps 3 & 4).

Thanks,

Gustav