They weren’t bugs back then, at least, not the first two. That was how the game functioned as intended at the time; Blizzard later made changes based on feedback, but that was later on, and therefore, not included in Classic.
The inconsistency in quest markings may or may not have been a bug, but quite frankly, it’s not game-breaking and just lends to the authenticity of Classic, which is Blizzard’s ultimate goal. Again, it was something they fixed/changed later on.
“Bugs,” in this case, would be defined as a conflict between the old code and current client. Something that should have functioned a certain way during Patch 1.12 that isn’t functioning that way currently. There are a few certain exceptions they’ve made in order to prevent cheating, etc., but for the most part, Classic is turning out to be very much like Patch 1.12 Vanilla, right down to the same bugs that were present during that time. Those are not bugs that Blizzard will fix.