We had the same issue and I think I found a workaround. The issue seems to be, that changes in language which you apply to the shop in general aren’t applied to all products.
In our case the default language was german. We added some products, lets call them XY. Then we added english as second language. Then we added some further products AB.
Exporting XY and AB always worked. Importing changes did just work for AB.
For XY we got the error:
FRAMEWORK__INVALID_UUID: Value is not a valid UUID: en-GB
Deleting XY, and recreating these products fixed the problem and we were able to import changes using csv.
This leads me to the conclusion, that something in the database was not created during language addition for all products already existing.
Hope this helps someone.
Edit for devs: Maybe this is already solved in newer versions of shopware (without updating to the newest). I did not try to create a minimum example with the current version, but most likely the approach described here will trigger the bug.
Best regards,
Dietmar