Is hyphenation turned off?
If so, and the text size is increased when editing you may find that things like web addresses, or long headings become too long to display across a single column. Increasing the width of text box may well allow what 'disappeared' to re-appear as the column width has increased sufficiently to accommodate.
I don't care much for hyphenation and will often add a soft return (shift+return) to force a 'non breaking' line of text to the next line to avoid this.
Beware of adding soft returns to to headings that may appear in a 'contents list' as the text will appear on two lines if forced over this way.
Thanks for reply Steve.
That's not the problem, however, as this seems to knockout multiple text boxes at once that were previously fine (so no extra-long non-breaking words to affect individual boxes).
It doesn't seem to be an issue with any standard sort of user error, this seems like a bug in my InDesign/setup/document. Curious that it's happened across two totally different documents and not others, though.
If the text is being forced into overset, turn on Hidden Characters (Type > Show Hidden Characters); do you see a tiny (could they make it any smaller?) "V" at the end of the visible text? That's a frame break, caused by pressing the numeric keypad Enter rather than the Enter/Return in the alpha keyboard. Replace with plain old Return or soft return.
Could it be, instead, a screen refresh issue? Are you in High Quality Display with Optical Margin Alignment activated? I'm working on a project with long chapters and tons of anchored graphics, and this was driving me nuts. In desperation, I turned off Optical Margins, and that has fixed it (so far).
Accidental application of a style involving [Paper] colored text?
An odd set of dimensions applied to a text frame in the Auto-Size options of Object > Text Frame Options?
I'm grasping at straws here... ;-)
Another thought on this issue (similar found through experience).
Is any of the text styled with 'keep' options that specify a start location eg next odd/even page?
If edited text reflows a couple of lines over onto the next, say odd numbered page, and there is text on that page with a 'keep style' set to 'next odd page' that text will jump through all the text boxes on that page until it reaches the next odd page.