Tridents can only drop from drowns that are holding a trident which have a 6.25% chance of spawning and only 8.5% of drowns holding a trident will actually drop the trident. Also make sure you are directly killing the mob or a tamed wolf is killing it. If you don't get the trident after killing about 13 drowns with tridents then this is likely a bug
I'm pretty sure this is a feature
Can confirm.
This is not a bug, this is a feature. Having two instances open at the same time causes no harm.