My textboxes keep vanishing in Pages: why?

I am typesetting a 150 page document that involves a lot of images (musical notation) and textboxes that accompany each image. For some ineffable reason, there are always a few textboxes whose text partially or entirely vanishes, thus rendering a pdf export or a print job impossible. It does this even if I don't make any changes. I can just scroll away and back and it happens.

Text wrap solves nothing.

I am at my wits end.

Why is this happening and what is the fix?