Hello.
Regrettably, this seems to be a bug in the current version. I'm very sorry for this inconvenience. We will try fix it in the nearest version and in the next hotfix package for this particular Kentico
CMS version.
In the meantime, could you please check what value you have in NodeHeadTags field in CMS_Tree table in DB for appropriate documents documents? Could you please set null manually to these fields. It should ensure proper behavior then.
Best Regards,
Radek Macalik