This is intentional, I believe. If you press the "Spellcheck" button in the editor, then refocus inside the content, the red lines appear.   Once you've corrected actual spelling errors (as opposed to invented ones, and @ article mentions) press the "Spellcheck" button to return to the usual editor.

Except the expectation is to /use/ browser spellcheck, per this suggestion: https://www.worldanvil.com/community/voting/suggestion/864cb5d1-69ea-4b97-b7ff-66fac4c6a270/view

I am really sorry but this is not a World Anvil bug,

@T. Mystery - And my Firefox's in-built spellchecker does work. In the editor, press "Spellcheck". The editor's code colouring vanishes - click inside it and then your browser's spellcheck kicks in. You can then see which are genuine spelling errors. Once done, press "Spellcheck" and the squiggly lines disappear and code colouring is re-enabled.   I'm on Linux, but I'm using the same browser as you, so I'm not sure how your spellcheck isn't working. As @dimitris said, it's not a WA bug, but as just another world-builder, I don't mind trying to help you get spellchecking working.