I believe the reason would be because Revit considers Demolition a part of the current phase (New Construction). Revit can only access past phases thru the "Existing" phase filter (no access to "Completed Demo", for example), and can't access future phases at all (no "Future Demo"). To show Demo, you must "look backwards" from the current phase.

So that ends up meaning that you're saying "the views for this phase are showing up in a view set to this phase", which is expected behavior. I typically have view filters set up for each of my elevation and section view "types" (Interior wall elevation, Ext Wall Section, Enlarged RR Elev, Demo Ext Elevation, etc), and place the appropriate ones in the view template. This method (name the view types, create filters, add to views) has become standard workflow for me.

You're right, thank you - I'd convinced myself this morning that phase filters would certainly filter out views that didn't correspond with the same phase filter and thought it was some sort of bug, haha. I'm not sure how I'd forgotten having used filters for elevations in demo view for this reason.