In a statement released late April 26, NASA’s Jet Propulsion Laboratory said a structure that is part of the heat shield for the Mars 2020 spacecraft cracked during testing earlier in the month at a Lockheed Martin facility near Denver.The fracture in the composite structure is located near the outer edge of the shield and spans its circumference, according to the JPL statement. It was detected April 12 after the heat shield underwent a week of testing at the Lockheed facility where it was subjected to forces 20 percent above those expected in a normal entry into the Martian atmosphere.
The article also specifies this was a spare heat shield constructed for MSL's campaign, so it is around 10 years old. Maybe the PICA was too "cured"?
Once the new structure is complete and tested, the thermal protection tiles will then be installed for flight, and the heatshield and other components of the aeroshell will be delivered to NASA’s Kennedy Space Center in Florida for final spacecraft processing prior to launch.