ASP.NET Core - Session not getting saved - session

Question

I'm trying to save my session in ASP.NET Core, but it is not getting saved.
I have looked at other answers, suggesting to change CookiePolicyOptions and nothing has worked so far. I have another project with the exact same code (presumably), and it works there but not in this project.
In my controller I have:
[HttpPost]
public IActionResult AddToPlan(int mealId)
{
PlanCart planCart = GetPlanCart();
planCart.AddItem(mealId);
SavePlanCart(planCart);
// ALWAYS 1
var y = planCart.returnList();
foreach (var x in y)
{
var z = x; // For debug purposes
}
return RedirectToAction("Index");
}
private PlanCart GetPlanCart()
{
PlanCart planCart = HttpContext.Session.GetJson<PlanCart>("PlanCart") ?? new PlanCart();
return planCart;
}
private void SavePlanCart(PlanCart planCart)
{
HttpContext.Session.SetJson("PlanCart", planCart);
}
I have a class with extension methods:
public static class SessionsExtensions
{
public static void SetJson(this ISession session, string key, object value)
{
session.SetString(key, JsonConvert.SerializeObject(value));
}
public static T GetJson<T>(this ISession session, string key)
{
var sessionData = session.GetString(key);
return sessionData == null
? default(T) : JsonConvert.DeserializeObject<T>(sessionData);
}
}
Startup class:
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Home/Error");
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseAuthentication();
app.UseSession();
app.UseMvc(routes =>
{
routes.MapRoute(
name: "default",
template: "{controller=Home}/{action=Index}/{id?}");
});
app.UseCookiePolicy();
}
services.Configure<CookiePolicyOptions>(options =>
{
// This lambda determines whether user consent for non-essential cookies is needed for a given request.
options.CheckConsentNeeded = context => true;
options.MinimumSameSitePolicy = SameSiteMode.None;
});
services.AddSession();
services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
services.AddMemoryCache();
I have checked my session. The session DOES exist but every time the count of the PlanCartList is 1 and previous items are lost.
If anybody could help me it would be very much appreciated because I'm losing my mind here!