It looks like when a recurring event crosses over from one daylight saving state to the other, the time of the event changes by an hour in the Upcoming Events calendar. This will happen even when the event and the user viewing the Upcoming Events calendar have matching timezones.
An easy way to reproduce this is to create an event topic with the following markdown (you may need to change the timezone value in the markdown to your own, or set your user’s timezone to America/Los_Angeles for the sake a reproducing the issue).
[event start="2024-11-01 10:00" status="public" name="Example" url="https://discourse.org" timezone="America/Los_Angeles" recurrence="every_week" minimal="true" end="2024-11-01 11:00"]
[/event]
Once you have created the event, go to /upcoming-events/week/2025/10/27 on your instance. The event will show at the correct time. If you move forward a week (the week daylight saving time ends), it shows the event as an hour earlier.
Thanks Joffrey!