This is a minor edge case (stable 3.1.1).
How to reproduce:
- Publish a topic that uses roughly the maximum character limit as per ‘max post length’
- Set ‘max post length’ to a lower value e.g. 50% of the previous value. The length value must be lower than the characters used in the topic published in the previous step.
- Go to the topic created and click the topic actions menu spanner → ‘Change Timestamp’
The topic will appear to update, but the date will stay the same and no error is given. I believe the sidebar scroll will show the first post at the new date iirc, but topic date will still show the previous date.
I think this case should either update the timestamp, or show an error (like when attempting to edit an existing topic with a revision that has a character limit above the max post length).