I am looking at a ticket IAD-JNB and on return CPT-IAD. I was thinking about buying business on the way to JNB and economy on the return. The business is a special fare so I know I cant pick seat but the economy is a flex. I should be able to pick my seat for the economy flex but it wouldnt allow it when I priced it out. If I pick economy seats round trip it will allow me to pick up my seats.

Is this a glitch or could I pick seat after I purchase the ticket?