I didn't do automation on the track. It turns out it has something to do with the midi files I use (they're from the internet). There must be some midi signal telling it to take the reverb level back some other position. Today, I noticed that also the "Master Volume" in pianissimo automatically jumped to 100% after pause/play. Must be in the midi file. Is there a way to bypass or overrule those midi commands?
A brand new project without a prefab midi does a proper job, indeed.