I’m not sure when this started, but it seems like %{base_path} isn’t being substituted in some instances.
The only place that I’ve seen this so far is the report a post prompt, where the url has the topic id replaced with %%7Bbase_path%7D/guidelines so https://www.example.com/t/topic-title/1234 becomes https://www.example.com/t/topic-title/%%7Bbase_path%7D/guidelines.
I tested and verified that this also happens in safe mode.
The text of post_action_types.inappropriate.description is the default:
This post contains content that a reasonable person would consider offensive, abusive, to be hateful conduct or a violation of <a href="%{base_path}/guidelines">our community guidelines</a>.
I tested changing the text while keeping the link the same. The changes do appear, but the issue is still there.
I checked another instance of %{base_path}, specifically site_settings.tags_listed_by_group which does appear to substitute correctly. The tags listed by group site setting does have a working link in the description.