Entities don't clear lists (like "ActiveEffects") before reloading from NBT.
If you remove the ActiveEffects tag and tell the entity to read from nbt again, the effects on the entity won't change.
This may have something to do with MC-2725 and could possibly easily be fixed by clearing lists the entity loads before it reads them.
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No response in 8 months, assuming no longer an issue.
Still an issue in 14w03b.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.