You’re right, that is very peculiar. It just tends to work.
I wonder if there’s some kind of funny cache issue going on, maybe turn off that page’s cacheing, or clear your site cache? That’s probably bunk, I’m grasping now. Maybe someone else might think of something. Might need to describe your system particulars. For instance, if someone’s on a Windows server, I know enough never to try to answer the question.
I have used anchors in HTML before, just never with MODx. It’s a usability preference, too - if a page is long enough to need those, to me it’s generally too long, and should be broken out into other pages. Just my opinion, and I agree with some exceptions, like big legalese pages that are nasty to read and find anything.
I have had clients who insist on mega long pages - their technical specs minutiae being very, very precious to them even if, of course, no one would ever read it. I figure, hey, I warned them.

Engineers and physicians just love to do this, it’s so funny.