The muscle feels tight. So you stretch it. It feels better for half an hour, then the tightness comes back, and it has been doing this for years.
Tightness is a sensation. Length is a measurement. They are not the same thing.
A muscle that is already stretched beyond its resting length will report tightness constantly, because it is under tension all day. Your upper traps with your head held forward. Your hamstrings with your pelvis tipped back in a chair. Those muscles are long and working overtime, and the correct response is to shorten and strengthen them, not to pull on them further.
Stretch them and you get twenty minutes of relief from the neurological effect, followed by a muscle that is slightly longer and slightly less able to do its job. Repeat for a year and the tightness is worse, not better.
How I tell the difference
I measure. Passive range on both sides tells me whether a muscle is actually short. If the range is normal but the sensation is there, the problem is control, not length, and stretching is the wrong tool. If the range genuinely is limited, we stretch, then immediately load the new range so it holds.
Range you cannot control is not yours yet.
“One of the most knowledgeable trainers I have encountered in 15+ years.”
This is the difference between chasing the symptom and finding the cause. The tight feeling is real. It is just not information about length.