asp.net Identity, Always remove user from the wrong role

Hi, i'm using asp.net mvc 5, I got 4 roles and some users in my database, there are 2 roles of them called purchase (name: PURCHASE) and production (name: PRODUCTION).

If a user got 3 roles (purchase, production, sales), when I try to remove this user from purchase role.....it was always successful. 

but the result is, production role removed this user, and purchase not......, I tried so many times in multiple ways, it aways has the same result....

is this a bug, or there is really something wrong with my code....

here is my code:

[HttpPut]
        [Route("api/User/RemoveFromRole/{roleName}/{userId}")]
        public async Task<IHttpActionResult> RemoveFromRole(string roleName, string userId)
        {
            if (roleName == UserConstant.AdminRoleName)
            {
                var user = await _userManager.FindByIdAsync(userId);
                if (user.UserName == "admin")
                {
                    return BadRequest("Cannot Remove Admin!");
                }
            }
            await _userManager.RemoveFromRoleAsync(userId, roleName);
            return Ok();
        }