I promise you, the Track from A to C, abeam point B, will be an exact Great Circle. It would be identical to Track from A to C without any intermediate points.
What may be confusing you is that on the LEGS Page, Initial Magnetic Track (IMT) is shown, and, as the Magnetic Track changes constantly throughout any Great Circle Track, the IMT abeam of point B will without doubt be different to the IMT at point A. Put in 10 Abeam waypoints on the same Great Circle Track, and you will see 10 IMT's
All of them are components of
ONE GC Track from A to C.
Another point which might possibly cause confusion is when you take a 'Direct To' with Abeam Waypoints, when the first Abeam Waypoint is quite close. The FMC computes Turn Radius for the first segment (that is to Abeam of the first waypoint) but not the entire Track. Thus, it is possible to see a kink in the Track abeam the first waypoint - The Fix? - Let the aircraft settle down on the new 'kinked' track, and re-insert Direct To the distant point with Abeam Points selected again, et voila!, the kink is gone
See you on line
Old Smokey