HI Amy,
When multiple people are co-authoring a PowerPoint presentation, the latest changes will be reflected in all users’ documents. For the problem you mentioned, there may be two reasons. One is that the document of the user who deleted the relevant content is
not correctly synchronized to OneDrive, and the other is that the PowerPoint client of the user who shows the previous version of the document has a problem when synchronizing with OneDrive.
To determine it, you may have these two users log in to OneDrive for the web and open the affected files with PowerPoint for the web to check whose PowerPoint client the problem is.
There is two KB articles for your reference:
Document collaboration and co-authoring;
Work together on PowerPoint presentations.
Best regards,
Madoc