Yes, it's an oversight when you look at it this way. But there is really no good way around it when you want to insert a whole new big thing into an established story.
It happens in fiction, e.g. even Star Wars is full of plot holes and continuity issues - this was true even when there were only the two sets of trilogies. (Obi-Wan not recognising R2, Leia remembering Padmé etc etc.) Now with all the side stuff they have produced, they have undermined the core of Star Wars, with Luke and Leia being the final hope of the jedi
And you don't even have to go to Andromeda, ME itself is really silly from the get-go if you think about it. The whole "uncovering Saren's betrayal" is just so full of rookie writing mistakes - the Council has no idea where their operative was at the time (and they give the victim's files to the prime suspect, lol); they are completely dismissing the testimony of an eye witness "because he was shocked"; no forensic method is applied to find out what type of gun killed Nihlus & whose gun it was; armed assassins attack Shepard on the Citadel (who is also armed while being a simple Alliance soldier at the time) and nobody thinks about following up on that; an audio recording that was obtained by a really questionable method is instantly accepted as damning evidence (how easy would it be to manipulate that?!) and also conveniently serves as an introduction to Benezia, whose line conveniently serves as an introduction to the Reapers - and of course the councilor instantly recognises the voice. I won't continue, you get the idea - ME is a great story because of its overall feel and likable characters, not because of its great writing.
Nothing good comes when you wind yourself up on these things and dissect them.
I don't see why would you start on this topic five years after Andromeda, nor do I understand why others try so hard to defend a really obvious issue.
