Cooling system problem?

[oldskewel]

2001 M54, 203k miles, head gasket job done at 186k miles, much of cooling system is already pretty new.

Recent sudden major problem(s):
- car otherwise runs fine
- coolant tank cap venting coolant after getting warmed up. This happened also when I had the HG problem a few years ago, fixed. Have done "block test" for combustion gases, which is negative, but I'm cautiously considering that inconclusive due to other issues that may mean the test is not valid.

In starting to diagnose, I took off the covers and air intake stuff for a better view. Did the block test, scratching my head so far, waited overnight for a better look, to re-do, think some more, and here is the real head scratcher I'm looking for help with:

Engine cold, everything off
Coolant tank cap (Rein, I think, 2 bar, a few years old) is off
Expansion tank (Rein, I think, barely a year and 6k miles old, fully intact, not leaking)
Upper Radiator hose (genuine BMW, 17k miles old) appears slightly collapsed as if it is under vacuum [spoiler alert - it still looks that way even after being removed, so whatever shape change happened got baked into it]
Lower Radiator hose (genuine BMW, 45k miles old) appears fine, not collapsed
So I'm figuring there is no pressure anywhere in the system due to the cap being off, puzzled by seeing and feeling the upper hose as if it is under vacuum, so I started to loosen the bleed screw [which is the highest point in the system] on the upper radiator hose, expecting to release the vacuum so air rushes in and the hose returns to its circular cross-section. But instead, the coolant is under pressure, forcefully coming out of the bleed screw hole, so I stopped removing it, wondering WTH is going on.

I have the 3.0i with automatic transmission, so here is the coolant flow diagram.



At this point, I'm just trying to understand how pressure can exist overnight, even with the expansion tank cap removed. Could a problem with the ATF thermostat at the bottom of the tank cause this condition?

I later took off a bunch of stuff, including the expansion tank, and the ATF thermostat is broken. So I'm definitely planning to replace that, but am hoping there are not more issues. Coolant thermostat (Genuine BMW, 45k miles old) and water pump (Saleri, 17k miles old) both removed and looking fine.

Separately from this (I think), I had got a Foxwell DME code indicating an off-spec cooling thermostat heater. I will replace that as well while I'm at it.

Also, the only "event" that could have triggered a change was that prior to this, my daughter drove the car home from SoCal, about 400+ miles, maybe faster than she should have. A good solid drive like that may have raised coolant and ATF temps higher than normal. But no overheating ever occurred. My daughter is very careful to notice that, and a few years ago I reprogrammed the coolant temp gauge to be mostly linear.