It may be a text wrap, try to select text frame then go to Object menu>Text Frame Options>General>Ignore Text Wrap.
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Probably better to remove the text wrap from the background if you don't need it for other text. And check the Text Wrap panel with nothing selected to make sure no wrap is set by default (and if it is, check with no files open to be sure it isn't a global default as well).
Both of you are correct, it was a text wrap issue! I must have accidentally left it on when working with a document element, and so when I created the master page background the text was trying to wrap, even if the image was on a lower layer. I hadn't even thought of that (and I also wrongly assumed that the image had to be on the same plane or above the text in question for it to wrap), but I'm glad I know now. Thanks!