Hi, I have an Alex Duetto from 2011. I've had it for 5 years now and it is not releasing pressure properly via the three-way valve.
Has anyone experienced the same problem here?
I am thinking that it could be due to not back flushing enough with detergent, but i am not sure.
It can make good coffee still, but when you turn the filter holder there is still a little pressure left

Most highly likely cam and pin wear, common all E61 groups. Cause exact opposite of what you suspect. Detergent back flushing without lubing the cam and pins afterwards accelerates the wear. Here's one thread talking about it.
Detergent Cleaning - E61 Manual Groupheads

Use the search function and you'll find other threads discussing E61 group maintenance etc. That's just the first relevant one I saw when I searched...