Fraser's brilliant cross from the left was tapped home at the far post by Archer as Southampton looked to have completed an unlikely turnaround.
The assistant referee did actually flag for offside, but it was extremely tight and VAR subsequently spent several minutes checking Archer's position, with the Saints forward eventually being ruled onside by the barest of margins.
However, that was not the end of it.
There was then a further check to decide whether Armstrong, who was clearly offside at the near post, had interfered with play as the ball came in.
The forward had overrun in trying to meet the cross, but flicked out a leg behind him, albeit not seeming to connect with the ball.
However, although the cross had already gone past Brighton defender Jan Paul van Hecke and goalkeeper Bart Verbruggen was rooted to his line, it was ruled that Armstrong's attempt to connect with the ball was significant enough for the goal to be ruled out.