Here’s a wild guess - might something be blocking the flow back to the radiator, but only one way? Don’t ask me how! Test by disconnecting hose at radiator and see if coolant will flow into a bucket on the floor. Only thing that reasonably could cause this (assuming system is tight) is a bad cap. Did you put a factory cap on?
I thought about that also, but I think another pressure test is in order. It could be a small leak that occurs at pressures lower than what is needed to send fluid back to the container. Give it 20 minutes on the pressure test, and if all is good it is something between the cap and the overflow container. OP says the flow is good.
I checked the engine compartment and could not find anything that pertained to the coolant level in the radiator. I also checked the manual and all it said was that the radiator should be completely full.
Yes the neck is actually a tube which extends down into the radiator. I have checked my brother’s 2001 Caravan and found the level in the radiator right up to the bottom of the cap, not 2 1/4" to 2 1/2" down in the tube like mine.
I did test the flow out of the overflow tank. First I completely removed all the anti-freeze from the overflow tank to be sure there wasn’t anything (goop or whatever) in the bottom that would block the return flow. Then I filled the tank fully with water and syphoned through the hose to a bucket on the floor. The tank syphoned completely empty.
The cap that was on the van when I bought it appears to be the original. The replacement I bought was an aftermarket Prestone brand (cost under $3). Dealer wants $13.
I have a prestone gas filter attached with a zip strip because things did not fit quite right, there is a plastic harness 1/8 of an inch smaller than fuel filter, as I said if the pressure test is good, it really narrows down the alternatives. At this point if it passes another pressure test it might be worth spending 13 bucks on an oem cap.
I hate to bring back such an old thread but am experiencing the same issue on my 2002 jeep grand cherokee 4.7. The level in both the rad and overflow are consistently the same. No overheating. Heat works great. Just the coolant level in the rad is always about 2" from the top. Did the OP ever solve this issue?