I updated the steps to reproduce. This may be more accurate, as it emphasizes use of the draftify script that we are seeing in our test cases.

Possibly related:

https://en.wikipedia.org/wiki/Special:Permalink/1117381183#Error_with_R2_tagging%3F

There's a bunch of redirects that reviewers have tagged [[User:Evad37/MoveToDraft.js|via draftify user script]] for speedy deletion under [[WP:CSD R2|CSD R2]] (inappropriate cross-namespace redirects) that can be found [https://en.wikipedia.org/w/index.php?search=Candidates+for+speedy+deletion+as+inappropriate+cross-namespace+redirects&title=Special:Search&profile=advanced&fulltext=1&ns0=1 here], yet none of them are included in [[:Category:Candidates for speedy deletion]] or [[:Category:Candidates for speedy deletion as inappropriate cross-namespace redirects]]. A few of these were tagged for speedy deletion up to 24 hours ago (e.g. [[Korea TV]] and [[Terry Gudaitis]]). [[User:Bennv123|Bennv123]] ([[User talk:Bennv123|talk]]) 02:03, 14 October 2022 (UTC)

Is there a bug with the edit API when using prependtext that doesn't fully process templatelinks or something?

In T321192#8335378, @Novem_Linguae wrote:

Is there a bug with the edit API when using prependtext that doesn't fully process templatelinks or something?

I don't see how that could happen, the edit API does not have its own parsing logic. It will just compose the page text from its various arguments and pass it to the parser.