Questions: TDC tutorial

[Von Due]

I just got Wolfpack and in all honesty, I haven't really learned to operate the boat...yet but I am a bit puzzled when I start the TDC tutorial.

At first, I had no bots, I looked around but within a few minutes the boat reached crush depth. I had not touched any valves at that point.
On 2nd attempt, I watched the depth meter as it showed the boat slowly sinking right from the start of the mission.

I played around with the main tanks but the sink rate was unchanged.
I then blew the trim tanks which had som water in them but the sink rate, if anything, increased.
I saw to it that the negative tank was empty and the bilge pump showed no water. Still the boat sank.
If I set any speed ahead, the boat would sink even more rapidly regardless of plane settings.

I then enabled bots and ordered to surface the boat. Nothing happened other than the boat sank yet again.

Is this a known issue or am I doing something terribly wrong?