As outlined in your post Why don't timestamps in the agent interface always match the UTC timestamp?, timestamps that were set outside DST / Summer Time are shown differently after the current timezone has shifted into DST/Summer Time.

Example: a ticket comment was added at 10:00 in December, during CET*, outside of Summer Time. At the moment of writing this forum post, we are in CEST**. When I view the ticket comment in Zendesk Support UI, it shows a timestamp of 11:00. I would expect to be shown 10:00.

* Central European Time, which is UTC+1 and extends (roughly) between end of October and start of April.
** Central European Summer Time, which is UTC+2