I do not know if this is an editor bug or an actual drag bug. If this is normal, please accept my apology.
I noticed I was getting a negative wave drag on a vessel I was building. After attempting to reproduce it in a clean, manually modded KSP install with just FAR 15.5.5, Flight integrator, and Module Manager, I was able to isolate the observed issue to these reproduction steps.
Start a new sandbox and enter the SPH.
Construct a simple craft featuring nose cones. For some reason, craft with flat noes are less susceptible.
Add radially attached tanks along the main fuselage.
Notice the extremely low, as low as negative, wave drag depending on where the side tanks are placed.