From the previously-linked Reuters story:
[. . .] Two people briefed on the matter told Reuters that an FAA test pilot during a simulator test last week was running scenarios seeking to intentionally activate the MCAS stall-prevention system. During one activation it took an extended period to recover the stabilizer trim system that is used to control the aircraft, the people said.
It was not clear if the situation that resulted in an uncommanded dive can be addressed with a software update or if it is a microprocessor issue that will require a hardware replacement.
In a separate statement, Boeing said addressing the new problem would remove a potential source of uncommanded movement by the plane’s stabilizer.