Would have expected them to emanate from the position of the thrown potion entity itself, as this is what happens with other thrown entities (snowballs, eggs, ...). Having it always appear at the center of a block would end up having them be generated at weird places when hitting entities.
Remember to make sure to not use floats!
Code analysis
Code analysis can be found in this comment.
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Comments 15
Code analysis (Mojang mappings, 22w15a): ThrownPotion#onHit() creates a level event when the potion hits something, using the block position instead of the entity position (LevelAccessor#levelEvent(...) requires a BlockPos):
...
int $$5 = $$2.hasInstantEffects() ? 2007 : 2002;
this.level.levelEvent($$5, this.blockPosition(), PotionUtils.getColor($$1));
...In LevelRenderer#levelEvent(...):
...
Vec3 $$25 = Vec3.atBottomCenterOf($$2);
...
SimpleParticleType $$30 = $$1 == 2007 ? ParticleTypes.INSTANT_EFFECT : ParticleTypes.EFFECT;
for (int $$31 = 0; $$31 < 100; ++$$31) {
double $$32 = $$4.nextDouble() * 4.0;
double $$33 = $$4.nextDouble() * Math.PI * 2.0;
double $$34 = Math.cos($$33) * $$32;
double $$35 = 0.01 + $$4.nextDouble() * 0.5;
double $$36 = Math.sin($$33) * $$32;
Particle $$37 = this.addParticleInternal($$30, $$30.getType().getOverrideLimiter(), $$25.x + $$34 * 0.1, $$25.y + 0.3, $$25.z + $$36 * 0.1, $$34, $$35, $$36);
...
}
...This uses Vec3.atBottomCenterOf(BlockPos) as the "reference" position to spawn the particles, which is (x + 0.5, y, z + 0.5), so the particles are spawned around the bottom center of the block.
This can cause potion particles to sink into the ground when thrown onto farmland, grass paths, bottom slabs, and other blocks without a solid top face.