Why do you use the PPTX format?
If you work with LibreOffice you should use the native format (ODF), for Impress this is ODP.
It could be a compatibility problem between Powerpoint and Impress.
Always create and save your files in LibreOffice and save them in ODF format (ODT, ODS, etc.).
Always keep these files as their source. If you need other formats for distribution to partners, you can open an ODF file and save and distribute another format with ″ Save as… ″.
This way, you always have working files available in your system environment.
Please report the behavior as a bug in Bugzilla .
See also: How to Report Bugs in LibreOffice
Please announce the link from the bug here.
To do this, edit your initial question. Thank you.