Assuming you're using Zotero 7, restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. Does it still happen?
If so, can you send the PDF in question to support@zotero.org with a link to this thread?
If so, can you send the PDF in question to support@zotero.org with a link to this thread?