This is not a bug, this is a feature. Having two instances open at the same time causes no harm.
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.
The best way to upgrade infdev worlds to 1.21.8 is to upgrade in small chunks of versions. Here is my recomended minecraft versions to upgrade to: infdev → Alpha 1.2.6 → Beta 1.7.3 → 1.8.9 → 1.21.8. Make sure to back up the world before doing this.