I didn't mention this in the original post because I wasn't sure when we'd be able to resolve it and I wasn't sure if this could cause additional problems, but it turns out that 2.20.0 changed how Essentials caches usernames and UUIDs. This wouldn't have been noteworthy if it wasn't for the fact that it ended up causing problems for us, because instead of converting the tried-and-true usermap.csv file, the Essentials developers decided to take the opportunity to read all player data and build the usermap entirely from scratch. You would have assumed that this approach would have worked, right? Right?
Wrong. This approach broke several commands related to offline players like /eseen and /home, because the conversion was incomplete and shitty and resulted in data either not being fetched properly or at all, even though user data is present in the folder. So, for the sake of documentation, this is how I fixed the issue. I created a very quick Java application that reads through the old usermap.csv file (which was used in EssentialsX 2.19.x and older) and converts it to the new format. Afterwards, all I had to do was restart the server and replace the usermap.bin with the one from the converter. The issues we were running into were instantly resolved.
To help those who end up in the same shitty circumstances as I did, I've put my converter on GitHub under the Unlicense license: https://github.com/VideoGameSmash12/ESUMC