Alt-text not exporting from InDesign to Acrobat consistently

I'm assuming this is an InDesign issue, but am happy to be corrected if it's Acrobat. Either way, the problem is that even though there is alt-text embedded in every linked image's metadata, it's not always exporting into Acrobat. The metadata for every image was edited in Bridge, saved, and the links updated so that's not the problem. 

And the alt-text is clearly there in the InDesign file:

But it always doesn't export with the PDF:

 Adding to the confusion is the fact that the Spanish language version is exporting the alt-text correctly:

 We used the English version as the basis for the Spanish file, so I'm even more baffled than ever. All of the images are grouped with captions then anchored in the text where referenced, the same for both language versions. They're all tagged as Figures, too, so there's no reason for Acrobat to be treating them like Artifacts.

Has anyone seen something like this before? Is this a problem within the InDesign file, a problem with the export process, or is it an issue with Acrobat?

Thank you in advance!