Where was the viable tissue found?
Mineralized soft tissue was found in the femur of a T. Rex in the Hell Creek Formation in Montana. The soft tissue was extracted and was found to contain vascular structures and cellular material. Here’s a link to a general science site that covers the finding, the initial scientific paper, and subsequent controversy: https://science.howstuffworks.com/environmental/earth/geology/soft-tissue-dinosaur-fossil.htm
Searching the subject will pull plenty of more in-depth papers and opinions.