When using 'ModifyEntity' on a mob or animal, it seems to immediately set the caster as the target's Magic-Owner.
This is especially troublesome when generating entities who self-cast ModifyEntity spells and are also meant to be tamed, as it means their 'Owner' is already defined as themself once they cast any ModifyEntity spell, and as such they cannot be tamed by players. I haven't been able to find a way to 'Untame' these mobs as a workaround either, as i cannot reset the owner, nor change 'Tamed' or 'Owned' to false, nor 'Owner' to blank.
(Image below is a Magic Mob who casts a ModifyEntity spell immediately upon spawning, then running the /data get command it shows the entity is owned by itself.)

That's awesome, thanks for the fast response!!

Hm I agree!
I can't see why it was setting ownership by default.

I've changed this for the next dev build (there's a new own parameter that is false by default)