I haven’t heard of this happening with the Delta cards, but I recently got a Marriott Bonvoy card and Amex told me that they hold all the points for 3 months before transferring them over (I assume to ensure I don’t get the promo and then cancel the card immediately). Iirc, the points were in a pending/to be transferred status too.

It’s possible they’re doing the same for new Delta cad accounts too and the Amex reps you spoke with didn’t know because the system didn’t tell the rep I spoke with when they’d deposit, he just knew since it was a new card and calculated based on the opening date.