The minecraft:spell_effects attribute can remove effects but not apply them. This is both true in its vanilla use with the zombie villager and trying to apply the attribute elsewhere using a behaviour pack.

To test:
Summon a zombie villager and throw a weakness splash potion at it to give it the weakness effect. interact with it using a golden apple to begin the conversion process to villager.

Result:
The zombie will shake, the conversion sound will play and the weakness effect will be removed, but the strength effect specified in the behaviour file is not added as evidenced by the lack of effect swirls and the fact that the zombie deals 3 damage to the player before and after the weakness and apple are applied.

Trying to apply potion effects to mobs as a component or component group will cause no effect to appear on the entity.