As it currently stands, there might not be enough evidence presented yet to warrant a tag (other than to keep an eye out on the accounts) but that does not mean there is nothing untoward going on. If/when anything of relevance is spotted, there will be multiple tags on both accounts.
Having said that, you have a valid point. From what I can recall, when members send PMs rather than engage in threads associated with their reputation it is probably because they do not want to bring more attention on their account as they are hiding other things they would prefer were not discussed.
Usually, someone who ignores threads like this and only sends private messages is an account hiding a lot and has at least one active alt account.
Therefore, it's best not to change anything, and I might add negative trust if they don't interact with this topic.
