I've been searching for a while and nothing helped me to solve this problem. I don't know what is happening, I used to send information to controller through a data object from AJAX using JQuery, but in this case, the parameter arrive null. My request look like this (I created a function based on button click):

Js Method on Razor page

const searchStudy = () => {
        let searchString = JSON.stringify({"searchString": $("#searchString").val()});
        console.log(searchString)

        $.ajax({
            url: "@Url.Action("SearchStudies","Studies")",
            method: "POST",
            processData: false,
            data: searchString,
            contentType: "application/json; charset=UTF-8",
            dataType: "html",
            success: function(response) {
                console.log(response)
                $("#dataContainer").html(response);
            }
        });
    }

Controller

[HttpPost]
public async Task<IActionResult> SearchStudies(string searchString)
{
    if (String.IsNullOrWhiteSpace(searchString))
        return BadRequest("Search something!");

    var studies = await _studyService.GetSearchStudies(searchString);

    return PartialView("~/Views/Studies/_Studies.cshtml", studies);
}

Ps.: When I try to use query data on url it works. Hope some of you have face this problem once :)