Transmission Not Go Into Gear problems of the 2016 Audi A6

Started vehicle in parking lot exited onto a multi-lane road where traffic speed is 50 mph but cars often drive much faster. The vehicle was in drive but stayed in first gear and would not automatically shift to higher gears. This caused the car to not merge at proper speed while vehicles were fast approaching. After multiple attempts at releasing and applying the gas pedal, the vehicle still failed to shift gears. Fortunately, I was able to find an area to pull off the road and after 15-30 seconds, the car behaved correctly. A similar situation happened a couple times while on the freeway where the car tachometer increased very high and the car failed to shift into higher gears. In these cases, I did not feel as unsafe as the vehicle was already travelling at 50-60 mph. I've taken the car to the dealer twice when the issues occurred but there are no diagnostics to indicate a problem occurred and the dealer was unable to reproduce the issue. I also informed the dealer that black liquid was leaking from the vehicle (see attached photos) but the dealer found no leaks. I photographed the leaks and saved a rag with the liquid and presented this at the dealer. There are no other vehicles or equipment that park in my garage and am 100% certain, the liquid is from the vehicle. The image shows that the leak had occurred several times and it persisted several times after the photo. However, I've not noticed the leak for the past couple months.