Ford didn't get much either after Saigon '75. He actually lost an election 18 months later. There was a minor uptick in the summer of 1975 in his approval rating (Saigon fell April 30), but that barely put him over 50%, as his approval was below 40% in that spring.
(Data from https://www.presidency.ucsb.edu/, plot: mine)
It's probably not reasonable to expect the effect to manifest over a withdrawal (even if that ends an unpopular war) coupled with the downfall of a former ally.
In general, the strongest rally-round-the-flag is seen immediately in the aftermath
of an event when an aggressor killed thousands of Americans (in that event) like 9/11 or
Pearl Harbor. While the Taliban killed thousands of Americans too, that was over the course
of 20 years... and no US troops were killed in Afghanistan in more than a whole year before this August. So there wasn't a close-enough timing. Also, the effect is usually seen when the president initiates action against the enemy.
Outside of those seldom-encountered circumstances, the effect [on initiating use of force or more generally when POTUS "vigorously responded" to an international crisis] is usually small, between 3% and 7%, according to one paper. So maybe Ford did see a "true" rally-round-the-flag, although Saigon '75 isn't discussed explicitly in the papers I've looked at (from this rally-round-the-flag perspective).
In general (same paper) the effect is more noticeable when the POTUS had low approval ratings beforehand (so that surely works in Ford's favor) and if there is bipartisan agreement on the policy/events. That actually seems to be lacking here a fair bit, regarding Biden: the withdrawal had more or less been agreed by Trump, but some Republicans have strongly criticized the final [mis]steps that they perceived in Biden's implementation. Fox News in particular has been absolutely trashing Biden on this, featuring Trump as one of the top critics as well.
As you're talking about ongoing events... nobody has a crystal ball, but NY Mag
expects Biden's rating to "muddle through" the fall of Kabul. I haven't seen any
pundits (I read) suggest he's going to get a large boost from it...
And with the benefit of a bit more time have elapsed, it looks like Biden's approval rating is taking a downturn due to Afghanistan... (about 4pp).
Americans seems to continue to approve the troop withdrawal from Afghanistan as a matter of principle, but disapprove of how it was done in practice.