Newly added event instance doesn't show up in list of custom / single events

Problem/Motivation

I have an existing event series of type "Custom/Single Event". I want to add an additional event instance. I do not want to remove and recreate all existing instances since I already have registrations for historic event instances. So I use "Add instance" to add the additional instance. This works great, but the new instance doesn't show up in the list when editing the event series.

Steps to reproduce

1. Create an event series of type "Custom/Single Event".
2. Add some dates and times under "Custom Date(s) and Time(s)".
3. Save the event series. Event instances for the entered dates are generated.
4. Now add another event instance on a different date using the "Add instance" local task.
5. Go back to the edit form of the event series.

Expected result: the newly added event instance is shown alongside the original instances.
Actual result: only the original instances are shown.