I ran into this same problem with an unlocked Samsung Galaxy S8. This problem has been around for years. My guess is that Comcast has a deal with Verizon to punish people that buy unlocked phones (since carriers prefer people to be locked in and only do unlocking at all because the FCC forces them to). I wouldn't expect it to work unless a new agreement is drawn up between Comcast and Verizon, which isn't very likely.
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