I dont know if this is common knowledge, but after hours of farming i can say 100% (at least for the recipes i was after) they are on a drop schedule (Channel based). Not sure if all items have their own set time.
Example: Monster Drops 1 recipe in its loot table at 200 kills, recipe 2 at 215, recipe 3 at 230, ect. (Ballpark numbers) Then it repeats. I Didnt see another recipe for another 200 kills and all 3 dropped in the same order they did before, the same kills apart from each other. This happened 3 times so i can say for sure the drops aren’t random. (No one else was in my channel killing them)
I farmed a second spot with a mob that dropped 2 recipes and they dropped in the same order on a similar schedule, 3 different times.
I dont like this because now i know if i am farming a recipe and im close to the drop kill # and some random guy walks by, he can possibly have the kill drop after i spent all that time. Should be a set drop %.
TLDR; Recipes drop on a specific kill # or spawn # (At least the 5 recipes i farmed)
If you want to try for yourself:
Go farm Kepa Raider Magician. Im putting these as if you just received the last recipe.
2 Blue gems will drop before the recipes begin to drop.
Roxona robe drops 1st
Roxona gloves drop 2nd
Roxona plate gloves drop 3rd.
Rinse, repeat. If you see the 2nd lv2 blue gem drop, the recipes are within about 25kills