The negative tank is only 4 cubic meters. It is intended to make you sink, hence 'negative' buoyancy. The trim tank is 15 cubic meters in size maximum. So if the trim tank is filled to the max, by adding 7-9 cubic meters (on top of the nominal starting level of 6-8 cubic meters) this is essentially the same as filling the negative tank and an extra one. This will make you sink anyhow if the ballast tanks are filled. Maybe slower if the neutral trimtank-level would be at 8. But none the less. Filling the negative tank on top of that only makes it sink faster.
The trim tank at the beginning should be close to the level that makes you neutrally buoyant when the ballast tanks are filled and have neutral diveplanes. Negative tank helps you dive a bit more quickly. As Stosstrupp indicated, for when there is an alarm situation and have planes or destroyers bearing down on you.
As for why you sank immediately in the tutorial mode is a mystery to me. Should not have happened unless you have a stowaway player onboard that messed up your tanks when you were not looking. Or accidentally ordered the dive bot to deep waters.
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