Added a note above regarding traces.
If we add a 10K resistor in series with the DMM + lead the glitch doesn't appear, a 1K or lower and the glitch still appears randomly.
This seems to indicate that the DMM Auto Range is inter-acting in some unique way, and maybe going thru ranges where the input saturates and this is disturbing the DMM input in such a way that's propagating out the DMM input, likely in a subtle charge, capacitance or impedance change.
Here's another trace at slow 50Hz BiPhase Modulation we got lucky to capture, note the glitch droop in the Zoomed in yellow trace.
One possible future solution is to include a monitoring buffer pickoff from the driver waveforms that includes a series resistor, may include this in a breakout PCB for the DMM and scope monitoring points.
One of the reasons we're concerned, is others will become involved with this advanced research project, and utilizing multiple DMM6500s to carefully monitor the channels. These DMMs could cause a disruption in the advanced MMW beam steering system which the multiple waveforms control, and that would be a nightmare to trace back to the DMM as the source.
Could not duplicate this result using a AWG squarewave, even including a series resistance.
Anyway, thanks for help.
Best,