I tried the most basic recreation. I just started a 3 column fixed with header and footer, changed the width and saved the preset. Then I created a new page using the preset. I was able to change the width of the page as you say and I could not recreate the error.
I'll try to track what the issue was if I encounter it again. When it happened, I had made a much larger number of changes to the page design, so it is difficult to recreate.
I also repeated this process with the layout that caused me the problem. The one where I went into the xml file to change and I could change the page width. I think I may have developed a cache error or something when I was working before.