Hi,
Not sure, but I don't think there is a risk of shortening, if it would be the case you would have already run into it causing damage.
The main problem according to me is that you are mixing stereo with balanced "protocols". Let me clarify.
The Monotron Delay is essentially a mono device. Left and right signals on its headphone output are identical.
Your balanced input DAC expects a mono signal, but transported as a differential signal: meaning twice the signal, once as a positive and once as negative version of it (XLR). Your DAC will look at the difference between the 2 signals. This makes XLR much higher tolerant for noise picking in on (long) cables (in technical terms: a high common mode rejection ratio).
So using your current connection cable, both same left and right signals enter as the positive and negative signal of your DAC. The difference of the left and right from a mono device is (almost) zero, hence you get the extremely low volume.
By pulling the patch cable back, you cut either the left or right signal and basically substitute it by the ground/shielding. The difference between left or right and ground is just the signal or the negative of it, hence loud and clear. You basically use your balanced input in unbalanced mode.
What you need is a this cable:
https://www.amazon.com/Cable-Matters-Ad ... V1L5M?th=1
It splits the stereo TRS into a left and right XRL. As the monotron is mono, you can leave one of the XLRs unconnected.
Have fun.