I want to track a user who has previously logged in so that I can associate traffic behavior with their user record on our system. Does storing the user's id as a single value in a cookie cause security issues?
It depends on how you validate the user id. Remember, cookies are user-supplied data, and should not be blindly trusted. If you want to save the userId on a cookie, make sure that you can detect when the user changed the cookie.
You could use JWT for that, so if the user modifies the cookie, the signature won't match and you can discard the cookie. Otherwise the user can impersonate anyone just by changing the cookie that he controls entirely.