SEOmatic plugin override global title in individual template
Are you using the SEOMatic Meta fieldType? 

What I do is create a field of this field type and assign it to the entry type you're dealing with. Within this field, there are all sorts of values you can override. For instance, you can point the SEO title field to one of your other fields via a dropdown, or you can manually type in a new SEO title just for this particular entry.