I had cleaned my seatpost wedge etc as @
Cyclefiend suggested but I still lost 1cm in 30km. So I took it back again.
I think mine has been dropping (only had it 2 weeks) when the isospeed has moved a lot, it does seem to be the back/forth movement that prompts it to slip but that coincides with hitting vertical drops in the road so it's hard to be sure.
I don't think anything holding the seatpost is under that plastic cover in your first photo. I don't think there's any reason to go inside there unless you're replacing the isospeed mechanism.
The only thing holding the seatpost up is the friction between the wedge at the front of the post and the inside front of the tube, and the two edges at the back of the post against the inside of the back of the tube. And that is done by tightening the two 5mm hex bolts (ideally using a long torque wrench attachment) in the slot at the back of the post which your second yellow arrow points at.
BUT that pressure/friction maxes out once those bolts are in as far as they can go. You can put as much torque on that as you like (don't try) and it won't push the wedge any further forward because they physically run out of thread. The ends of the bolts just go into two threadless hollows on the back of the wedge to force it forwards.
Apparently Trek told my retailer to cut off two washers from those bolts which then allows them to push the wedge a little further (using the recommended 7Nm). This is obviously a slightly insane engineer's response to a design or manufacturing tolerance failure but they've done it anyway.
I'm going to test it today. (edit: I am reviewing this plan in light of the massive hail storm that just began)
... As an aside, I've spoken in person to 3 Madone 2019 owners - of a 6 disc, 7 disc, and 8. None of them have had this problem and two of them are heavier than me.