It appears that the black box problem, and perhaps the disappearing problem, may be related to my saving the PDF images in Acrobat as reduced sized files [added: not so; see below]. In the first of two documents, the images appeared correctly using LO 5.3.4 when I used files saved as PDF/X-1A:2001, although the large LO file became much larger (almost 22 MB).
However, in the second similar document, but with more large images, all PDF/X images were no longer in the document after I saved and reopened the file, just the empty image boxes. I suspected a file size problem. [added: This document is 392 pages long, with two small CMYK PDF images on the first two pages and four large images toward the end. No problem with the first two images (I can save and reopen and those two images are there). The problem occurs after I add the four images. I tried adding them as grayscale .png. The two PDF images were not saved after I saved and reopened; the four .png images were saved. If I change the two PDFs to greyscale .pngs, they are saved properly. The grayscale images were about 1/4 the file size of their PDF versions.]
I’m running Mac OS 10.11.6 (El Capitan).