Doctor Who: Flux performs a hefty rewrite on UNIT's origin, but does this Brigadier retcon solve the Fifth Doctor's infamous UNIT dating controversy? Debuting in 1968's Second Doctor episode "The Invasion," UNIT quickly became part of Doctor Who DNA. An Earth-based military force designed to combat extra-terrestrial activity (calling The Doctor, in other words), UNIT has been a reliable Doctor Who ally, ever-present across the decades and traditionally led by Nicholas Courtney's Brigadier Alistair Gordon Lethbridge-Stewart. Doctor Who: Flux's "Survivors of the Flux" reframes UNIT's formation completely, revealing the villainous Grand Serpent constantly oversaw the organization from its initial conception to Chris Chibnall's off-screen dissolution in Doctor Who season 11's New Year's special.
Doctor Who: Flux also finds time for a touching Brigadier tribute. As the Grand Serpent and General Farquhar wander the halls of UNIT headquarters, a voice is heard calling out, "Lethbridge-Stewart here, I want a call to the RAF, please." It's a wonderful nod to the late Nicholas Courtney, who accompanied multiple regenerations of The Doctor and sadly passed away in 2011. The Brigadier's posthumous cameo in Doctor Who: Flux creates a plot hole, however. The scene takes place in 1967, but according to Second Doctor episode "The Invasion," Colonel Lethbridge-Stewart didn't join UNIT and become a Brigadier until the 1970s. Furthermore, the classic story heavily implies UNIT as an organization was formed around the same time - not 1958, as Doctor Who: Flux would have us believe.
One possible explanation is that Chris Chibnall's UNIT retcon is actually intended to address Doctor Who's infamous UNIT dating controversy from the Fifth Doctor era. The episode "Mawdryn Undead" shows the Brigadier retired in 1976 which, for the reasons described above, comes before he even joined UNIT. The plot hole has been endlessly dissected by fans since the episode aired in 1983, and even Doctor Who writers have admitted there's no working explanation to account for the Brigadier retiring from UNIT before he's recruited. While many fans gave up on Doctor Who ever resolving its UNIT dating controversy, Chris Chibnall and "Survivors of the Flux" clearly had other ideas, because the Grand Serpent may be responsible for both the new Brigadier plot hole and the old Fifth Doctor one.
Maybe in Doctor Who's natural timeline, Colonel Lethbridge-Stewart fights the Yeti in the 1970s ("The Web of Fear"), and subsequently gets promoted as UNIT's first Brigadier in time for the Cybermen invasion ("The Invasion"). However, the Grand Serpent might've altered this timeline to ensure UNIT is founded earlier (in 1958), and Alistair Lethbridge-Stewart joins while still a corporal, before "The Web of Fear" happens.
As a Doctor Who fan, Chris Chibnall is very likely aware of the "Mawdryn Undead" UNIT dating controversy, and retroactively fixing it is exactly the type of thing he'd attempt. If so, perhaps the Brigadier retired in 1976 because the Grand Serpent changed his timeline from the Second Doctor era. Whatever the reptilian villain wants with UNIT, he evidently needed to shift the organization's entire history approximately a decade earlier. And if Corporal Lethbridge-Stewart joins UNIT 10 years before he should've, it stands to reason he would retire sooner too, sick of all the extra-terrestrial stress. That could mean "Mawdryn Undead" takes place in a timeline affected by the Grand Serpent's meddling, and Doctor Who: Flux has finally plugged a plot hole that stood unresolved for almost 40 years.