New to DL and I hope I'm not asking a question answered many times before.

Trying to book a connection PHL-MSP-BIS round trip in Comfort+. The MSP-BIS legs do not have C+, but PHL-MSP do. The seat map for the PHL-MSP flight shows no C+ availability when flight 1 of the connection to BIS, but looking as a simple PHL-MSP booking, C+ seats are shown on the seat map of that flight. Same coming back. Comfort+ seat map shows wide open for MSP-PHL when just an MSP-PHL booking, but when part of the BIS-PHL itinerary there is nothing. Is this a quirk? Is there something about the connection that blocks the seats?