Using v7.15.4 and using tags set as csv and in content pre save event I am doing:

string tagPropertyAlias = Alert.GetModelPropertyType(x => x.ServiceNumber).PropertyTypeAlias;

        if(content.HasProperty(tagPropertyAlias))
        {
            //its stored as json array but cannot get it back as array so have to use string manipulation
            var serviceNumbers = content.GetValue<string>(tagPropertyAlias).Replace("[","").Replace("]","");

            if (!string.IsNullOrEmpty(serviceNumbers) && serviceNumbers.Contains(","))
            {
                var badTags = serviceNumbers.Split(',');
                List<string> goodTags = badTags.ToList();

                //do we need to re add to tag manager??
                content.SetTags(TagCacheStorageType.Csv,tagPropertyAlias,goodTags,true,"default");
            }
        }

I am doing this because I want users to be able to paste csv list of values see where if you do this then you get funny results see https://our.umbraco.com/forum/using-umbraco-and-getting-started/102387-cut-and-paste-many-tags .

So I have done it this way instead and it saves into content and it appears correctly in backoffice however the relation ships are not there, it seems as though set tags is not adding the relationships.