Mail Merge Saving vs Synchronous

Mail Merge Saving vs Synchronous

We've started having a problem recently when creating a Mail Merge from Zoho CRM that when selecting anything over about 50 records instead of 'Synchronous Merge (Download/Store/New)' we end up getting 'Merge & Save as files' instead.

Nothing has changed with the merge template, and we used to be able to merge 100 records at a time.  It's also started creating issues for anyone that doesn't have Writer or Workdrive.

I know we have sufficient credits, and even when looking at the downloaded file for 100 records it's only around 25,000 characters which I believe is below the limit.

How do I determine what is causing the merge to change into a download rather than just popping up in a new tab?