What do you mean by “move an XLR … to Mix 8”?
Your local XLR OUT 8?
But that should be patched to the right channel of your Stereo Mix 7 (also on your QU-7) by default (see image).
Therefore, this output would also work for your Mix 7.
However, you can patch your local outputs however you like.
So I have a female XLR plugged into Output 1. This is going to a headphone amp on stage. If I select Mix 1 on the desk, I can adjust the monitor mix for that person by adjusting fader levels. That is working fine.
But, if I plug that same XLR into say Output 8 and then select Mix 8, no signal is sent to the headphone amp.
You explained it perfectly, and that’s how I understood it as well.
But to reiterate: By default, your output jack 8 doesn’t have Mix 8 connected, but rather the right channel of Mix 7, as marked in the image.
Your stereo Mix 8 will be connected at OUT jacks 9 and 10.
But, as already mentioned, you can configure everything in the output patching as you wish - including having the left or right channel of your Mix 8 connected to jack OUT 8, if you would prefer.
Every jack is freely assignable.
Just so you know, you can go to Setup → Config → Mix Stereo and toggle mixes 7-12 to mono, if you prefer. (1-6 are mono and can be joined in odd-even pairs, 7-12 are mono or stereo.)