I have this same issue as well at all of my firefighting air stations.
I've purchased the additional hangars for smoke jumper aircraft and the slots are added, however, when going to the aircraft market, I cannot actually purchase any smoke jumper aircraft as the option just isn't there. Instead, it seems to be adding more slots to buy water drop aircraft.
I reported this in the support chat both on the web and on the phone app days ago but haven't received any responses.