I had just bought shadowkeep on PC, i just transfered (enabled crossplay) from xbox, and it says I do not own the season pass even though it's included with shadowkeep. I bought the DLC on steam not xbox as well.
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Sounds like you activated the pass on your PC account and any characters there and not on your XBL characters. Remember the season pass is only for one account and will not apply to others due to game share or cross play.