The behavior is the same as the bug listed here: Category setting to enable solutions doesn't stay checked.
For easy reference I copied and pasted that issue description below:
- Go to a category where people can mark solutions & click the Edit button to modify category settings.
- Notice “Allow topic owner and staff to mark a reply as the solution” is not checked.
- Check the box for that, and click Save Category.
- Open the edit dialog again.
- The box accurately represents the state of the toggle, and stays checked.
- The box is not checked for categories with solutions enabled, and the box never stays checked after checking it and saving settings.
Note that although the checkbox does not stay checked, the solution plugin is enabled (the functionality works) when the box is checked and ‘Save’ is clicked. However I can’t disable the plugin for a category after it’s enabled (I think this is related to the bug with the checkbox).