When you create an NPC in the campaign dashboard, you can select their current location. However, this choice does NOT carry over to the Character Article, and does NOT appear on the location page you have selected. It does work in reverse - if you create a character article in the world, and select a location, then the location article will update with "Characters in Location" section.
Campaign Dashboard > create supporting cast member Additional Details > Current Location. Pick an existing location article Return to World Dashboard > View World Navigate to Location Article > No characters are listed (BUG) ALTERNATIVE METHOD World Dashboard > create character article Under content > Template Specific Prompts > Current Location. Pick an existing location article Return to World Dashboard > View World Navigate to Location Article > Character in Location listed in side section (CORRECT IMPLEMENTATION)
Additional Info - support from u/SparkyOndo on the subreddit. https://www.reddit.com/r/WorldAnvil/comments/18bl6kj/npcs_in_world_how_to_organize_articles_not_being/?utm_source=share&utm_medium=web2x&context=3