Seems like 'posts' works okay, but it's 'comments' that don't (and overview is a mix of both).
API call to see posts:
curl --request GET --url 'https://lemmy.world/api/v3/user?username=beebarfbadger&sort=TopDay&page=1&limit=1' --header 'accept: application/json' | jq .posts[].post.published
=
2024-09-26T15:35:33.998368Z
(today's top post is today)
API call to see comments:
curl --request GET --url 'https://lemmy.world/api/v3/user?username=beebarfbadger&sort=TopHour&page=1&limit=1' --header 'accept: application/json' | jq .comments[].comment.published
=
2024-03-03T05:09:45.255807Z
(this hour's top comment was in March)
They probably know about it, but if not it's probably a good idea to report the bug here: https://github.com/LemmyNet/lemmy/issues