problem
Negative correlation means the left and right channels are opposing each other enough to risk mono loss.
Stereo correlation moves negative when the left and right channels contain enough opposite-polarity or heavily phase-shifted information.
This can come from stereo wideners, chorus, delay, doubled parts, phasey microphones, or low-end content spread too wide.
A brief negative dip on a wide effect may be harmless, but sustained negative correlation on drums, bass, or vocals is worth investigating.
Switch to mono and bypass stereo processors one at a time. The source that disappears, thins out, or shifts strangely is usually the issue.
Reduce widening, adjust polarity or timing, filter the sides, or keep low frequencies more centered when mono compatibility matters.
Meter Core links correlation and mono checks so phase decisions stay tied to what listeners will actually hear.