The reason the lcd would not work as intended on the side you drew it on is because the LCD needs a potential (voltage) difference between its two leads. Having it on the same side as the xt90 anti-spark plug, there would be zero potential difference because the xt90 is essentially just a wire connection. As such, there would be no change of potential at that end. However, the LCD would have a potential difference on the side with a load (VESC). The Vesc will has a significant resistance and will draw current. Thus, on the positive lead of the VESC there is the full 10s voltage and on the negative lead there is zero (the actual point of zero voltage in a circuit is arbitrary, but i chose that in this example).
In short, on the xt90 end there is no voltage drop for the lcd to measure, whereas on the other side, the VESC causes a voltage drop that can be measured.
To further clarify things (hopefully), say for example you have just a battery with positive and negative leads. you could connect the lcd to both leads and it would work.
Now connect the vesc in parallel with the lcd, like this:
Battery+ -------> vesc and lcd -----> Battery-
and it will still work. But, connect the vesc and then the lcd/xt90 in parallel:
Battery+ -------> vesc -----> lcd/xt90 -------> Battery-
and the lcd wont work
TL:DR The problem comes from connecting the lcd in parallel with something that effectively is just a wire so the lcd does not have a difference in electrical potential. I hope this helps.