I tried summoning a zombie with strength 2 using this command:

/summon zombie {ActiveEffects:[{Id:5b,Amplifier:1b,Duration:10000}]}

But it did normal damage, then I tried using a splash potion and it worked.

I also used

/data merge entity @e[type=zombie,limit=1] {ActiveEffects:[{Id:5b,Amplifier:1b,Duration:10000}]}

but it also had no effect, the particles showed up on both cases but they did normal damage.
The same happens with speed, weakness and slowness. Invisibility, fire resistance and slow falling seems to work fine.