EDIT 2: Yeah, it seems that unfortunately they started purging all the data just as I made this post. So for future reference, if you spot any of your favorites going missing, you can try this method to recover it, unless they specifically hard-coded their site so that this can't ever happen again.

EDIT: Well, as of literally right after I made this post, it seems they've started fixing this and the search method currently doesn't work: https://twitter.com/nhentaiofficial/status/1755022504496898360. Talk about unfortunate timing.

There is still a way to recover the codes from stuff you've favorited that were recently lost in the purge.

Step 1: Go to your favorites, and search for "*" (this will return every single result).

If you had a lot of favorites, (and due to it, lost a lot of favorites) the first page of results will probably look something like this:

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https://preview.redd.it/nerrzn1a72hc1.png?width=1321&format=png&auto=webp&s=701808436f19b717f162849ad6f6285dafecbc2f

There are a bunch of "empty" containers that have doujins that were recently deleted. The deleted ones should show up first in the results, before any existing links.

Step 2: Right click anywhere on the light grey area surrounded by the "Remove" buttons, and click "Inspect".

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https://preview.redd.it/9w574oga72hc1.png?width=483&format=png&auto=webp&s=cae0e655902e6dee7427c56b145e77c93a3b9086

You should be prompted to a div with the id "favcontainer". Expand this div, or if it's already expanded, you should see a list of div classes titled "gallery-favorite".

The data-id is what you're looking for here. That's the code of the doujin that's supposed to be there but isn't. If you search that code on the internet, you should be able to find your lost doujin.

For example, take that second one on the image above, with the data-id "203346". If I google search "203346 hentai", I'm able to find pictures and the title of that hentai (which btw, was Souma Taxi by Mizuryu Kei).

Repeat this for every results page where you see empty containers like this.

I recommend doing this as soon as possible. Who knows how long it'll take before that data is completely removed from the site.