Reddish tint with spectral renderer
Is it reasonable to expect the image of the described scene to be "in grayscale"? If so, do light intensity spectra indeed are required to be multiplied by D65 spectrum? 


My expectation from a physically-based spectral rendered is that it would


compute the physical spectra
use the CIE color matching functions to relate that to a quantified color space (eg CIEXYZ)
convert that into the destination color space.


D65 shall not appear anywhere till step 3, and only if we actually render into sRGB (or similar).

A flat spectra translates to XYZ = (0.33, 0.33, 0.33), which is RGB = (1.20, 0.95, 0.91) prior to gamma compression. So the "red tint" is the expected result following the above procedure.

Don't know why Mitsuba would have such a 'feature'.