If the text pane is narrow, no text appears, because the text would either be set to a ridiculous line width or would be partly obscured and thus illegible. This is intended and in fact a good deal of effort has been devoted to polishing this; if your Mac is sufficiently powerful, for example, the text pane will gradually fade as you narrow its compass.
But that’s not the problem described here, which has been reported twice but is clearly quite rare. What’s happening here is, apparently, a problem in which the text pane is hidden as if no note was selected, even though a note is selected. We’ve not yet been able to reproduce the issue.