Tridents not being acquired by non-player killing methods.

I noticed that my drowned farm on a server acted a bit weird after I afk'ed throughout the night (around 9 hours), I saw that I only had gotten nautilus shells, gold, emeralds and rotten flesh. However there were no signs of ANY tridents at all. The farm had worked quite well before I tried to automate it by letting drowneds fall down onto magma blocks.

As I saw this issue I tried to find out why it did that. So I went onto the testing server and tried spawning around 128 drowneds (wheares around 10 of these had tridents) in a big magma-blocked area in order to avoid EntityCramming. Not a single trident got dropped from this. After that I proceeded to spawn 128 drowneds WITH tridents, and still no luck. After that I tried alot of other various ways of killing drowneds, such as using lava, suffocation, death by another mob, fall damage, cacti, anvils & thunder. It still doesn't work. They still only dropped when I killed them by myself.

After that I was getting a tad confused to why it didn't work, so I spawned a Drowned, with 100% chance of dropping it's trident. It worked when I killed it by myself, but not when I tried any other non-player killing technique.