Symphony Nitris (non DX) v3.0.

Working with NTSC 29.97 anamorphic downconvert from 1080p 23.98.
Resolution is uncompressed SD.

I did all my color correction with the monitors set to 16:9, and got
everything set. Then, before output to tape, switched back to 4:3
monitors and applied a 16:9 reformat effect to the top track.
Suddenly, all of my clips showed peak gamut errors for low RGB.
No amount of tweaking could get rid of it, but if I clicked the
monitor icon to the track below the reformat effect, everything looked
fine.
Anyone else seen this? Any ideas?