Lol weird...
But I had a simmilar issue when using easy touch asset (not sure if it will apply here, but maybe).
I had all touch buttons set up to respond to pressing space (by default). Had some very strange behaviour lol, when i actually pressed space- every button would be activated at once- but the actual touch buttons worked flawlessly (when used on screen and when space is not pressed).
I guess what im saying is that, maybe you should explore the entire 'ui stop button' in the inspector to ensure it is not being directly stimulated by the space key. (I attached an image of my jump button, that was messing around ((the bits that were causing the issue are highlighted in pink)), bear in mind mine is in easy touch, but your one may have something similar somewhere in the inspector for your GUI-button.)
I hope it helps (otherwise i can only guess that it may be a complex setting in inputs that is beyond my understanding)...