Hi Mercedes,
I suspect that this might have to do with the default installation culture of your various versions, the language of the interface when your descriptions were created (i.e. the source culture), and some known issues in AtoM that have to do with exporting non-English CSV data.
These bug tickets outline the issue that I think you are encountering:
There were also some previous reports about this in the user forum, such as:
The previous workaround suggested was to run your exports from the command-line, where all rows in all cultures were included in the export by default.
If you can upgrade to 2.8.1, the Maintainers have made changes there so that the CSV export from the user interface should now behave like the CLI export, and include all available rows in all cultures on export. 2.8.1 release notes:
As to why 2.6.4 behaved differently:
This is just a guess, but: is it possible that the default installation culture of your site was different then (i.e. left to the default EN installation culture, instead of ES)? And/or, is it possible that archivists were creating Spanish descriptions, but while in the English user interface?
If so, it's possible that this might explain the different behavior. Nothing changed in AtoM 2.7 regarding how AtoM handles exports, as far as I know, and the bug tickets linked above were first reproduced in 2.4 (meaning they affected 2.4, 2.5, 2.6, and 2.7.x releases).
It may be worth doing a bit of investigating to see if the language of your descriptions matches the source culture that AtoM thinks they are! Unfortunately, this does sometimes happen in AtoM when people add descriptions in one language, (e.g. Spanish) but the AtoM user interface is set to English - so AtoM thinks the descriptions are in English, not Spanish.
This can be a really complicated issue to fix, so it is best to identify it early if possible! I did try to recently provide some initial guidance to another user facing a similar challenge here:
You could potentially use the SQL SELECT statement examples provided in that thread to check your own data and see how many rows AtoM thinks are English vs Spanish across the main entity tables.
Cheers,