Libre Impress not showing .pdf after a while

I have Libre Office 40m0(Build:2) on Ubuntu 17.10

My LibreOffice Impress keeps deleting all the .pdf images I insert to it every so often (can’t figure out the periodicity yet). It looks like this when it happens… it only happens to the .pdf files, all the .png that were inserted are fine. Besides happening every so often out of nowhere, it also happens when I try to export to a pdf with the images looking fine before and not afterwards (and also not visible in the pdf) and when I close the file.

I have increased the memory by a lot, and have gotten rid of the autosaving feature some other questions blamed. My memory is now as this.

Neither of those two solutions have fixed my problem.
I would apprecciate a solution to this problem. Thanks!