The messages are between the MAX’s then-chief technical pilot, Mark Forkner, and another Boeing pilot, the sources said, and raised questions about the MCAS’s performance in the simulator in which he said it was “running rampant.”
For non-native readers it may be useful to explain the meaning of “running rampant”: To go unchecked or without control; to be wild or excessive.
https://en.wiktionary.org/wiki/run_rampant