[Peter T. Daniels]

I inserted a picture (a bitmap), cropped and sized it to fill the page
from side to side, and positioned it 1.5" from the top "margin" so
that it would be at the top of the text area.

I then clicked on Insert Caption and got the usual panel offering
"Figure 1" as its label. Instead of simply inserting a paragraph in
Caption format as usual, it inserted a text box with the caption label
in it. When I typed the content of the caption, the following body
text reduced to a single line. Dragging the picture's anchor to a
different paragraph made whole paragraphs appear or not on the page --
and also made the caption disappear.

I right-clicked the text box to see if it had a "space after" setting,
or some such, and an option was "convert to frame." When I did that,
the body text came back to fill up the bottom of the page.

Why would a text box prevent all but one line of text from appearing
below it?

(All the keep-with options are unchecked -- next, together, and widow/
orphan.)