Damage hearts are not shown when damaging with sweep attack

Can confirm for MC 1.12.1.

Confirmed for 1.13.1.

affects 20w22a

Can confirm in 21w05b.

Can confirm in 21w06a.

I am able to confirm this behavior in 21w41a. Here are some extra details regarding this problem.

The Bug:

Entities damaged with the sweeping attack indirectly don't produce minecraft:damage_indicator particles.

Steps to Reproduce:

• Obtain a netherite sword with the sweeping edge enchantment.

/give @s minecraft:netherite_sword{Enchantments:[{id:"sweeping",lvl:3}]}

• Summon two creepers next to one another.

/summon minecraft:creeper ~ ~ ~-2.5 {NoAI:1b}
/summon minecraft:creeper ~1 ~ ~-2.5 {NoAI:1b}

• Damage any of the one creepers with a sweeping attack using the sword.

• Take note as to whether or not the creeper that you didn't directly damage produces minecraft:damage_indicator particles.

Observed Behavior:

Entities damaged with the sweeping attack indirectly don't produce minecraft:damage_indicator particles.

Expected Behavior:

Entities damaged with the sweeping attack indirectly would produce minecraft:damage_indicator particles.

Can confirm in 1.18.2.

Can confirm in 1.19.

Can confirm in 1.19.2.

Code analysis (Mojang mappings, 1.19.4 Pre-release 3): The attacking heart particles (DAMAGE_INDICATOR) are added in Player#attack(Entity), but only for the directly attacked entity. If this is a bug, I believe a possible fix is to change this part of the function:

...
                if ($$9) {
                    ...
                    List<LivingEntity> $$18 = this.level.getEntitiesOfClass(LivingEntity.class, $$0.getBoundingBox().inflate(1.0, 0.25, 1.0));
                    for (LivingEntity $$19 : $$18) {
                        if ($$19 == this || $$19 == $$0 || this.isAlliedTo($$19) || $$19 instanceof ArmorStand && ((ArmorStand)$$19).isMarker() || !(this.distanceToSqr((Entity)$$19) < 9.0)) continue;
                        $$19.knockback(0.4f, Mth.sin(this.getYRot() * ((float)Math.PI / 180)), -Mth.cos(this.getYRot() * ((float)Math.PI / 180)));
                        $$19.hurt(this.damageSources().playerAttack(this), $$17);
                    }
                    ...
                }
...

to something along the lines of:

...
                if ($$9) {
                    ...
                    List<LivingEntity> $$18 = this.level.getEntitiesOfClass(LivingEntity.class, $$0.getBoundingBox().inflate(1.0, 0.25, 1.0));
                    for (LivingEntity $$19 : $$18) {
                        if ($$19 == this || $$19 == $$0 || this.isAlliedTo($$19) || $$19 instanceof ArmorStand && ((ArmorStand)$$19).isMarker() || !(this.distanceToSqr((Entity)$$19) < 9.0)) continue;
                        float healthBefore = $$19.getHealth();
                        $$19.knockback(0.4f, Mth.sin(this.getYRot() * ((float)Math.PI / 180)), -Mth.cos(this.getYRot() * ((float)Math.PI / 180)));   
                        $$19.hurt(this.damageSources().playerAttack(this), $$17);
                        float damage = healthBefore - $$19.getHealth();
                        if (damage > 2.0f) {
                             int amount = (int)((double)damage * 0.5);
 ((ServerLevel)this.level).sendParticles(ParticleTypes.DAMAGE_INDICATOR, $$19.getX(), $$19.getY(0.5), $$19.getZ(), amount, 0.1, 0.0, 0.1, 0.2);
                        }
                    }
                    ...
                }
...

which would be in line with how the attacking particles are added later in the function.