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.
Comments 2
@greggo Are you certain that the zombie was given the strength effect? As I described, the weakness effect will be removed but the strength affect was not applied. It is recognised as a bug by MCM-147.
I would also like to mention that spell_effect is able to remove effects from other entities I tested it on, not just zombie villagers which suggests that the component supports other entities but the add-effect parameter is broken.
When I tested with the vanilla behavior packs on the zombie villager, it seems to be working fine. However, it seems that it isn't implemented yet for anything other than that specific use in the vanilla behavior packs. It looks like that component is only working on zombie villagers, and I don't even think it will allow other potion effects besides the ones vanilla uses for the curing behavior. Unfortunately we have quite a number of components that only work on specific mobs right now and are basically still hardcoded.