"Show on task" previews suddenly not showing

This has worked fine up until last week. We have several planners with PowerPoint documents attached to the task cards. Up until last week we could see the previews on the task card. Now, none of the previews are showing for any of the users. Just the PPT logo. This is happening both in the Teams app (Mac) and browser.

If the task is viewed on a mobile device, then the preview shows as expected.

Other image file types such as JPG etc. still work OK.