I have reported this before, but It seems just a bit too weird. Raku and I have noticed that we aren't getting any custom enchants when we enchant anymore. When I reported this before, Jordan got Venom 6 pretty quickly when he tested it, but Raku and I have enchanted a good amount of books since then (I myself have enchanted a couple chests worth of them) and still haven't received any custom enchants. About a year ago I used to get a variety of low level CE and some higher / better ones from enchanting way less books from what I have done now. It just doesn't seem like we are extremely unlucky at this point (but we may be still THAT unlucky), as it seems more of like a permissions or 1.13 issue.
@Chocofuzz What are you trying to enchant? I Tried this on an alt account and I was still able to get a custom enchantment on a level 30 book nearly instantly.
ive been trying to get beheading specifically, but I haven't enchanted any custom enchants in a long time. I will try again with like 100+ levels shortly.