Try it with a reminder (created in Reminder app) for next Monday 25.05. Then the reminder disappears in (in Agenda) after changing the title (in Reminder).
Strangely with a reminder for Saturday (tomorrow) or Sunday it does not happen (the renamed reminder stays visible in Agenda).
I did this on MacOS, but I could see the same behavior in Agenda on iOS (but did not create/change reminders in the Reminders app on iOS).
Two videos are on the way to you.