Did same job on my 300TD. No problem at all. Even old tranny seal looked as good as new one.I think its something else.
I dont know on your car but on mine, the rubber seal is held in place by rubber clips( which are part of the gasket itself).
Once held in place, put all screws in and hand tighten all of them a few threads, slowly tighten the bolts on the pan across from each other. staring with the corner ones. While I was doing this, I was holding the pan with other hand in place. The trick is you want to make sure that the tightening happens uniformly all across the pan surface and in small increments.
I did not need any sealent. I did not use torque wrench, but went underneath the car after few days and check the tightness of all bolts again.

My feeling is that you have an uneven or damaged pan surface of the gasket is torn. Give it a good look.

MVK

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1985 300D Turbo 179,000miles