Recently, a change in either the model Kagi uses for this mode or an upstream model provider causes most attempts to translate NSFW images to fail when the mode is set to Best.

This only affects the Best mode, so Standard still works.
This mode used to be fine with translating it, and it still manages to do so (albeit rarely, possibly when it picks a different model?). Translating NSFW content does seem to be an intended feature judging from other related issues, so this seems to be an unintended change.