The reason the compromise position is not always correct is this:
The two things are not opposites in a way that works with a percentages.
For example: What percent pregnant are you? What is the middle ground? Can we compromise so that we are each half pregnant?
It’s in those cases that it’s a fallacy. What can be tricky is recognizing that the two things presented “as if” opposites on a spectrum are not actually on a spectrum and they might not even be opposites! Pro-life and pro-choice is an example of issues that are not opposites of each other but only seem to be.