I recall previously that when a post has commentary (like from the original source) and you fetch commentary from a URL, it adds that commentary with tns for each one, but now, when I try to do that, it overwrites the current one entirely, for example on post #5387636. Is this a deliberate change or what? If I want to have multiple commentaries, would I need to do so manually now?
The reason it's not working on the post you mentioned, it that the Plurk commentary would be in the description, whereas the Pixiv one is in the title with no description. There's a bug where if the title is empty in the commentary on Danbooru, but the source you're fetching from has a title but no description, it'll add the title and overwrite whatever is in the description box with a [tn] source link.
There are several other bugs besides the one I mentioned above, so you just need to be wary when using that function.