Postpositive "concerned": temporary state of affairs
The Cambridge Grammar of the English Language reads
Postpositive present (or absent) denotes a temporary state of affairs: compare the present government. The same applies to involved and concerned, though here the attributive sense differs more (cf. deeply involved activists, concerned parents).
However, oxfordlearnersdictionaries.com does not seem to include the second meaning as an adjective
CONCERN: [often passive] to affect/involve somebody/something: Don't interfere in what doesn't concern you. The loss was a tragedy for all concerned (= all those affected by it).
What does temporary state of affairs mean in this context? How does it distinguish the postpositive concerned?
Your reading of CGEL's text needs to be revisited. Let's add back some context:
[12] ii the people present, the cars involved, the students concerned, the city proper . . .
The adjectives in [12ii] occur both attributively and postpositively, but with a difference in sense. Postpositive present (or absent) denotes a temporary state of affairs: compare the present government. The same applies to involved and concerned, though here the attributive sense differs more (cf. deeply involved activists, concerned parents).
Now let's delete the intervening sentence about a temporary state of affairs (which pertains only to present and absent):
The adjectives in [12ii] occur both attributively and postpositively, but with a difference in sense. . . . The same applies to involved and concerned, though here the attributive sense differs more (cf. deeply involved activists, concerned parents).
I think you can see that what is being said (though not terribly clearly) is that concerned has a different sense depending on whether it is used attributively or postpositively.
Now look up concerned in some other dictionaries to understand the difference in sense CGEL is getting at:
The concerned [caring] students freed the zoo tigers.
The students concerned [implicated] were expelled after the freeing the zoo tigers.
The zoo tigers concerned [involved] were seen wandering the neighborhood.
Concerned [worried] neighbors are staying inside.