Thanks to the answer of @user535733 and a Discussion I had with some people in the group of #KUbuntu in Libera some days ago, now I think have the answer to my own question:
Ubuntu philosophy is to live away GNU ecosystem, but using GNU ecosystem.
Allow me to explain: If KDE for example, moves to 5.26 and leaves 5.24 for LTS, Ubuntu decides to be careless about KDE decisions (that are the real maintainers of such package) and prefer to downstream patches to the version they have to settle down (lets say 5.25 or 5.23 or whatever version they are at the moment of the settlement)
Their argument is: Ubuntu aims to keep offering super stable versions for every single package. This could make sense in LTS Ubuntu versions, and it would be even wiser to be aware of the official package maintainers LTS decisions and stick to them (sometimes they do and sometimes they don't).
But their mindset is to stick to whatever version they decide, regardless of the upstream decisions, something that is weird because many packages have lost maintenance (for example KDE 5.25) and now all new patches are being implemented in 5.26. They argue that the latest version is less stable that 5.25 because it's newer, but since it their primary patch focus, it always leads faster to an stable position that the previous, now unstable version (in this example 5.25) because they will be solving issues in 5.26 that still occurs in 5.25 (because ultimately each piece of software doesn't matter if it evolves the version, it will carry many issues from version to version).
But Ubuntu maintainers, have to do a double-effort to patch 5.25 with some of the patches they find in 5.26 KDE bug tracker. And they have to do for every-single-package!!! Ubuntu maintainers are doubling the job of hundreds, if not thousands of packages. Its completely nuts.
And all of this gigantic effort just to live in an illusion of stability, and proof is exactly this one: I keep finding bugs that higher versions of certain packages have already solved, but Ubuntu maintainers have not patched yet downstream in their theoretically lower and more "stable" versions (which are no more stable because they have more bugs than the upstream ones).
To sum up, they create non-official versioning of each single package, like what is happening from ages with PHP.
Considering that PHP has like 20 modules, one module like in this current case, FPM, happens to step in a lower version forget to move up versions of certain extra packages, and we end with a system with 12 packages held up indefinitely and having to tweak the upgrading systems to avoid constant unneeded warns and infos that could be an important thing to read in certain real situations.
Some users could say that this is an answer-rant.
But this is a little explanation to myself, to put a little context so I can understand better why the real solution for this is technically what I said in the former question: simply use a better repository for each package. In this case, PHP repo has been a synonym Ondrej since the beginning of the times.
And a side not for me, in case I stumble into the same question in a couple of months (I have a bad memory). For KDE and the liking, I should always stick to Ubuntu backports repo that has the upstream versions generally.
