This issue seems to of crept back up.
T4MVC OptionalParameter values implied from current context
The divisionteamid is being populated from the calling page even though I don't explicitly state it. If I change the parameter to teamid, it works and doesnt generate a URL with divisionteamid or teamid in this matter.
public virtual ActionResult Registration(int? id, string slug, int? divisionTeamId = null, int? divisionId = null)
return GetRegistration(id, divisionId, divisionTeamId, new EventRegistrationViewModel());
<a href="@Url.Action(MVC.Event.Registration(Model.Event.Id, Model.Event.Slug, null, null))">Registration</a>