Morning!
I confirm the behaviour, using dt 3.3.0+1217 — however, I am not certain about it being a bug 
I am on some thin ice here, but let us try to analyze the image:
Look at the histogram (before any changes). The image has an RGB value
of 231, 136, 136, resulting in two spikes in the histogram. One R spike (value 231), and a combined G/B spike at value 136.
Then you invoke the mid-tones hue eyedropper in the color balance module, which manages to put the spikes closer together – but it does not succeed in neutralizing the image (in which case all three spikes would have ended up the same spot).
So this leaves me with a hypothesis that the red original spike is outside the work range intended to be taken care of by the mid-tone eyedropper.
“Proof”: instead of moving the saturation slider, invoke a second color balance module, and click the mid-tones/hue eyedropper again. Since the spikes already were quite close when you invoked the second color balance module, they now are in the proper working range.
Or something similar – unless, of course, I have misunderstood the whole thing!
Have fun!
Claes in Lund, Sweden