How to apply image frame fitting consistently across all records in Data Merge (InDesign)

I just had this same issue but managed to figure it out! With the @image selected in the Data Merge tab, open the Data Merge menu and select “Content Placement Options...”. Setting the Fitting here will apply it to all entries. In contrast, clicking on the image in the editor and changing the Fitting properties from there will get reset every time you change a record. 

But then when navigating between records in the data merge, this setting is reset and the scale of the image changes back to default.

I'm having a hard trime recreating this, to be honest. I can set the "Fill frame proportionally" preference in the frame to which I assign the @IMAGE placeholder just as well as I can select it in the Options tab of the "Create Merged Document" dialog. Frames are filled proportionally whether I'm previewing records or just scrolling through the merged document. 

Can you perhaps post a sample file, or a video? 

After selecting "Fill frame proportionally" in the frame fitting options, the image fills the frame correctly, without leaving whitespace in the rectangle. But then when navigating between records in the data merge, this setting is reset and the scale of the image changes back to default.

When you are navigating between records, are you using the preview buttons? I honestly don't trust the preview - it has had known issues for a long time.

When you say the "scale of the image changes back to default" what was the default?

If I were in your chair troubleshooting at your desktop, I'd make an Object Style for the frame meant to hold the image, and I'd assign the preferred frame fitting option in the object style. Then, after I ran the merge, I could check all of those frames and see if any overrides had been applied by the merge process. Or, if the image aspect ratio changed but no override was applied to the object style, I could see if manually changing the frame fitting options would actually change the aspect ratio or fitting options, in order to prove that there was a bug with the frame fitting options.