This will happen with facing pages and two-page masters when you add or remove an odd number of pages, causing existing pages to change sides of the spread.
In InDesign should NEVER be a dummy text on a master. InDesign is not Quark Xpress, where this method made sense. It is a complete different program, so please forget behaviors from Quark Xpress workflows. They are here completely wrong.
If you put text frames on the master, they only should be for automatic place holders like page numbers or running header. It is best to put them on their own layer.
If you work with CS6 or newer you can also use primary text frames on the master. In older versions I would also recommend not to put any empty place holder frame on the master. (Ok, I know some exceptions, where I do it myself, but this is a different topic.)
InDesign uses the margin and column guides to build up inserted text frames.
If you need text frames exactly on the very same place again and again, don't use place holder, but create a new INDL InDesign Library file and put it there as object. If you place these items via command—not by dryg and drop—it will remember exact position and—if this setting is turned on in the Layer Panel's menu—their original layer.
Avoid overriding master page element on pages if you can. Master page's objects are NOT part of a content in an exported PDF, only artifact, which might be important if you need to create PDF/UA. If you decide to export any EPUB without fixed layout, Master Page objects will be ignored completely. But if you override them, they become part of content. So I would strongly recommend to create several Master Pages as "Children" of the first Master and override those elements there to be consequent in design and structure of a document.
Thanks Willi. I am indeed an oldschool Quark guy, though been using InDesign for years now. I'll look into the INDL method - which i don't even know about! - but if it's the right way, I'll learn.