nowdays i can see, and play against, many youngsters who are really great in tactics, they see motives, they execute combinations like top class masters, when it comes to positional, when game is calm down and demands proper, deeper, strategy they fall down in just few moves. particularly noticed this at regular club otb blitz and rapid tournaments.
Yes, it's common. What is also common is some of these youngsters go on to become GMs, whereas we do not. So we should take whatever points we can get now, while the taking is good.
So, there's a theory I've heard put forth by a number of chess educators and high-level players about the value of puzzles being mainly training for pattern recognition. In this model, the value of puzzles is far less to practice slow, deliberate calculation (a skill set that for many people seems to not improve very rapidly with practice) but instead to practice doing easier puzzles faster.
The goal, ultimately, is to move increasingly complicated tactics out of the realm of calculation and into the realm of instantly recognizing board positions that lead to them. The hope would be that in real games, these pattern recognition skills would replace calculation and make finding tactics easier.
I'm not at a level of experience or skill to be able to judge this idea independently, but I will say that starting to practice puzzles this way has led to seeing certain things in real games that I would not have seen before.
RE: the value of puzzles being mainly training for pattern recognition
I wouldn't say mainly. That's one use of puzzles, but they can be used to train many different skills in chess, provided they are well selected for the skill being trained, e.g. pattern, depth (look-ahead), breadth (short-term memory), imagination, etc. In fact pattern and imagination are at odds with one another, pattern is something you have seen before, and imagination is thinking outside the box.