Hello Giuseppe,
I’m absolutely and definitely no expert, but I’d just like to share a strange experience with you in case it helps. A few months ago I moved to a better instrument (cornet) with a third-valve trigger. I wanted to see what difference pulling the trigger made, so I played a D with varying degrees of trigger with my tuner. The strange thing: the trigger actually made no difference whatsoever! I knew it just had to; the pipe was getting longer, so pitch should have gone down!?! Then I realised that I was compensating for the trigger with my embouchure. Something in my head had decided to try and keep the note constant, irrespective of how much trigger I used. I wonder if the same might be happening to you. You have a note in your head and no matter what you do with the tuning slide, your subconcious is always aiming for that note. Is that possible?

Jutta