old airbus would remember a left turn to MA heading where you originally set it and turn flight directors the wrong way! Boeing knew to take the shortest path to the heading preset. Airbus did not. One more friggin distraction!
It's a distraction because you learned it one way first and there is very little standardization between manufacturers. As pilots whatever we've seen first we tend to view as 'right' and 'smart' and anything else is just awful. What you say is correct about the A300 heading bug, but it's also true of many other aircraft including the 737 from what I remember(been awhile). The shortest path thing has gotten people into trouble as well with the aircraft turning an unwanted direction. I know on the 727 you have to be careful with big turns on the A/P because it starts the turn so slowly that if you don't move the heading bug incrementally, she'll reverse course on you. As someone who flew planes that wouldn't do this before flying the 727, I could call it a distraction but really I just view it as how that particular airplane does business. Relatively speaking (before the experts jump me), I think the only thing that is sort of standard between manufacturers are the FMS because they tend to come from a very small group of vendors.
You also had to check and see that the MA was still in the FMS, IIRC. Sometimes it just didn't stay in there with the runway change.
Are you talking about a runway change in the box or are you talking about talking about it disappearing as you circle to land on the opposite end of the runway in the box? In the latter case, it's not all that surprising to me because the circle in most cases will overfly the the MA procedure in the opposite direction which will confuse many FMS' causing them to drop those points out. That can happen on a lot of FMS'. I seem to remember some drawing tons and tons of white/magenta circles on the map as it tried to figure out what you were doing on a circle. Some could figure it out and 'straighten' the circles out when you engaged TOGA (although it was a leap of faith looking at it!) others couldn't and you knew to use Heading Select and raw data.
All part of the fun. It is what it is, and as always you just have to know your plane and think ahead of it modern or not. Some like to make this a young vs. old thing, but I've seen both have problems with it. From what I've seen most older guys seem to understand the basic systems (elec, hyd, air, etc) better and more in-depth than younger guys, probably because that's how they were trained initially and that's where most of the problems with older a/c are. When it comes to those systems, which often have as many quirks and things that need massaging as modern ac fmc/autopilots, those pilots tend to think of those issues like a fine wine and hammer those who do not understand the quirks well or complain about how they should be fixed. However that doesn't extend to the A/P FMC on modern a/c where the same pilots will often spend half the flight telling you how the thing should be redesigned and is a crime against humanity. The younger guys tend to accept the FMC/AP for what it is, sometimes turning their brains off in the process, but have little interest or knowledge in how the other a/c systems work. They're comfortable with the automation and they know the 'routine' very, very well. Where a lot screw up to me is that they don't apply that sometimes natural comfort with the automation with the older guys zeal for understanding how the system really works and what could go wrong. Worst is that many don't seem to care. While I'm still big nag on keeping your handflying skills up in an FMC/AP world, overtime I'm beginning to think that lack of true understanding of what the system is doing beyond following the profile is almost as big of a factor in many incidents/accidents. Most of the main systems on modern a/c are pretty reliable making the old school overanalyzing them not as fruitful imo, but the same cannot be said for the automation which is more complex than it often seems on the surface and becoming increasingly so. Most of the time the answer to 'what's it doing now?' is that we didn't
really understand what it was doing in the first place. Very broad brush strokes for sure on young v. old before I get jumped on the above.