Bug or balance / design, hard to say. Anti-air options (other than fighters for intercept), I just noticed that Anti-Aircraft guns which open at Ballistics, go obsolete at Rocketry just like in the vanilla game. Normally that's when Mobile SAM opens up. Here, we wait til Advanced Ballistics, many techs later. This means AA guns kind of briefly pop up, go obsolete, but can't be replaced until a lot of techs later. Only noticed in a game I'm playing where I noticed the neighbours fielding bombers, thought "better get AA going" and didn't see the option anymore. On my way to Advanced Ballistics now... but again I thought "premature obsolescence is probably an oversight." For the little its worth, the tooltip for Rocketry still talks about Mobile SAM.