Just to clarify SearchPage is my custom object, not a cms page.

Looks like problem is inside ExistingActionRouteConstraint constraint.

Reflected code:

public virtual bool Match(Route route, SegmentContext routingContext, string parameterName)
{
    IContent content;
    string str = routingContext.RouteData.Values["action"] as string;
    if (string.IsNullOrEmpty(str))
    {
        return true;
    }
    if (!this._mapContentRouteParameters.ContentLoader.TryGet<IContent>(routingContext.RoutedContentLink, LanguageSelector.MasterLanguage(), out content))
    {
        return false;
    }
    TemplateModel model = this._mapContentRouteParameters.TemplateResolver.Resolve(HttpContext.Current.ContextBaseOrNull(), content, TemplateTypeCategories.MvcController, routingContext.ContextMode);
    if ((model != null) && (model.TemplateType != null))
    {
        return this._actions.GetControllerActions(model.TemplateType).Contains(str);
    }
    string a = (route.Defaults[RoutingConstants.ActionKey] as string) ?? "index";
    return string.Equals(a, str, StringComparison.OrdinalIgnoreCase);
}

It does action lookup not on actual routed data, but on "parent" content.

routingContext.RoutedContentLink

Is it a valid behaviour?

If I add "second" (with same name) STUB action on "parent" cms-content controller, it starts working:)

Looks like a bug.

Here is workaroung I've found:

    public class Global : EPiServer.Global
    {
        protected override void RegisterRoutes(RouteCollection routes)
        {
            base.RegisterRoutes(routes);
            //go through content routes and apply constraint fix
            FixExistingActionConstraint(routes);
        }

        private static void FixExistingActionConstraint(RouteCollection routes)
        {
            foreach (var route in routes)
            {
                var contentRoute = route as ContentRoute;
                if (contentRoute != null && contentRoute.Constraints.ContainsKey(RoutingConstants.ActionKey))
                {
                    var original = contentRoute.Constraints[RoutingConstants.ActionKey] as ExistingActionRouteConstraint;
                    if (original != null)
                    {
                        //replace constraint with custom wrapper
                        contentRoute.Constraints[RoutingConstants.ActionKey] = new ExistingActionRouteConstraintWrapper(original);
                    }
                }
            }
        }
    }

    public class ExistingActionRouteConstraintWrapper : IContentRouteConstraint
    {
        private readonly ExistingActionRouteConstraint original;

        public ExistingActionRouteConstraintWrapper(ExistingActionRouteConstraint original)
        {
            this.original = original;
        }

        public bool Match(Route route, SegmentContext segmentContext, string parameterName)
        {
            //try to minify "fix" influence and process only "my" partials routes (objects are marked with IVirtualPage interface)
            if (segmentContext.RoutedObject is IVirtualPage)
            {
                return true;
            }

            return original.Match(route, segmentContext, parameterName);
        }
    }

Does it make sense? Any pitfalls?

Hi and thanks for the detailed explanation!

I agree that this should be considered a bug. That is if a partial router has routed to something the validation should be against that controller (if any) and not the "original" cms content.

I have reported a bug for this.

Fixed in 7.6.5, release note: http://world.episerver.com/Documentation/Release-Notes/ReleaseNote/?releaseNoteId=112923