Why are supposedly ActivityPub-federated Discourse threads inaccessible via external AP clients?

Example

If I enter [https://meta.discourse.org/t/uris-to-the-origin-of-a-thread-that-has-since-been-merged-should-503-to-the-merged-comment/342417?u=rokejulianlockhart][1] (or [https://meta.discourse.org/ap/object/bc8f042ccca5ef56734c5f2460aa45cc][2]) [^1] into [mastodon.social/search][3], it 404s: [^2] [^3]

If this isn't Discourse's fault, I'll take this to Mastodon. However, most other AP platforms that I've tried (that I've expected to work) have worked.

[1]: https://meta.discourse.org/t/uris-to-the-origin-of-a-thread-that-has-since-been-merged-should-503-to-the-merged-comment/342417?u=rokejulianlockhart[2]: https://meta.discourse.org/ap/object/bc8f042ccca5ef56734c5f2460aa45cc[3]: https://mastodon.social/search[4]: https://mastodon.social/search?q=https%3A%2F%2Fmeta.discourse.org%2Fap%2Fobject%2Fbc8f042ccca5ef56734c5f2460aa45cc#:~:text=Posts-,No%20results.,-Explore[5]: https://mastodon.social/search?q=https%3A%2F%2Fmeta.discourse.org%2Ft%2Furis-to-the-origin-of-a-thread-that-has-since-been-merged-should-503-to-the-merged-comment%2F342417%3Fu%3Drokejulianlockhart#:~:text=Posts-,No%20results.,-Explore

[^1]: [https://meta.discourse.org/t/uris-to-the-origin-of-a-thread-that-has-since-been-merged-should-503-to-the-merged-comment/342417?u=rokejulianlockhart][1][^2]: [search?q=https%3A%2F%2Fmeta.discourse.org%2Fap%2Fobject%2Fbc8f042ccca5ef56734c5f2460aa45cc][4][^3]: [search?q=https%3A%2F%2Fmeta.discourse.org%2Ft%2Furis-to-the-origin-of-a-thread-that-has-since-been-merged-should-503-to-the-merged-comment%2F342417%3Fu%3Drokejulianlockhart][5]



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