jcb1973 @Ando or @DavecUK - do you have anything you can maybe share from Lelit acknowledging this issue and how it needs to be fixed, to help us deal with retailers who may not know about this or be unwilling to help?
My thoughts, or best guess…as I identified the component that was the issue, what was causing it as lubrication alleviates it, albeit temporarily for many people. I believe there are a couple of suppliers of this part according to lelit and I think one of them has “improved” things by closer tolerances. It’s why all machines are not affected
In this image you can see a brass disk with a rubber/viton pad on the end…this pad presses against an opening to seal it. When the pressure is high enough, it pushes the pad open against the spring force and allows excess pressure out.
I believe something may be slightly off centre inside the expansion valve and this is causing the disk to tilt slightly as it goes back and jam, until it gets wet inside….then it doesn’t jam so easily. The mark on the pad even looks slightly off centre. The manufacturer of the expansion valve probably isn’t aware of it, because they are usually further from the boiler and tend to stay moist inside. I would imagine this expansion valve dries inside.
It can easily be solved by a slightly different choice of expansion valve, there are many types, including spring and ball types. I suspect they are just trying to find the best one.