Steps to reproduce:

1. Add a field of type ezkeyword to the 'article' content type;
2. Create a new article, fill in all required fields, and publish;
3. Edit and publish the article 5 times, so that there are more than 5 archive versions (default value of default_version_archive_limit);

Ref: https://doc.ez.no/display/DEVELOPER/Repository#Repository-LimitofarchivedContentitemversions

4. Edit the article again, and this time add a keyword to the new ezkeyword field. Publish;
5. The keyword was not saved. Notice that it is not present in the article 'Details' tab, and if you edit it you'll also notice the field is empty.

Note: according to the customer, this is the relevant piece of code in ezpublish-kernel:

https://github.com/ezsystems/ezpublish-kernel/commit/bd8178660eb2824d387b9c64217b1d9fa9ef2e6#commitcomment-20963103