Around October 12, T9551 made project hashtags stricter and prevented them from containing characters like comma (,).
Around December 27, D14888 changed how hashtags queries work so that the query does normalization instead of requiring the caller to normalize.
After the Dec 27 change, projects from before Oct 12 with now-invalid hashtags will no longer load when queried directly by hashtag, because the page queries for old,[silly]hash,,tag or whatever, it gets normalized into old_silly_hash_tag, and then there are no hits.
Instead, at least for now, query by both the exact raw text and the normalized hashtag. This should keep older stuff working until we can give users more support for migrating forward.