Hi,
The reason is, you have used the submit button(as below), which is uses the Post HTTP verb. Please do use the Anchor tag(link) or Input type button to navigate to the login page and outsource you can present the Anchor tag as button using CSS.
<button type="submit" class="btn btn-primary" asp-area="Identity" asp-page="Register" id="RegisterButton">@_loc[Model.RegisterButtonName]</button>
Happy coding!!