Whenever I use the "minecraft:behavior.delayed_attack" AI goal for my mob to do melee attacks. My melee attacks that were defined by "minecraft:attack" involve giving Poison to the victim. When the melee delayed attack takes place, one would expect the victim to receive the damage and the Poison status effect, but the victim only receives the damage, but not the Poison status effect. 

Used code:

To define the melee attack:

"minecraft:attack": {
	"damage": 2,
	"effect_name": "poison",
	"effect_duration": 7
}

To define the delayed attack goal:

"minecraft:behavior.delayed_attack": {
	"priority": 3,
	"reach_multiplier": 1.5,
	"hit_delay_pct": 0.5,
	"attack_duration": 1.4,
	"track_target": true
}