Thanks for the reply @tomas_hora although not very useful. As you can see from the graph in my original post, the Xp and Ti parameters are correctly set. Stepping up from 2 to 3, the PI controller works fine. Stepping down from 3 to 2, the behavior is strange and unexpected. Neither changing the parameters nor switching to PID control solves the problem.
Also your reply about the B34 and B35 blocks does not bring any useful information. I can see the heating/cooling description in the title of the block. I'm trying to control pressure. Which one should I use then? The documentation is the same for both blocks.
Please explain. Thanks.