It's not a bug. You aren't changing a segment, you are changing the ticket because changing any segment, changes the ticket. What that does will depend on the circumstances.

As noted, you may certainly wait until the outbound has been flown as that cannot then be repriced, but there is no guarantee of what will be available at that point.

I would recomend that you call UA. There may be alternatives which work here and do not significantly change the price, e.g. flying through EWR (p.s. service on the tcon).