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From the official website. Teratypes for pokemon from raids are determined by the raid crystal. Whether or not the crystal changes types over time is not clear. I assume the crystal raids will be like SWSH raid dens and each crystal will have a set list of available pokemon that is not all pokemon. This could indicate that not all pokemon can pick up all types BUT that there are going to be lots of weird types.
This also gives GF room to do seasonal events putting special pokemon in raids so that you can get them in any type the way you can get special raid pokemon from any den during the events in SWSH.
