Invalid activation token - problem with controller?

I have a really weird problem. Everything worked in previous version of .net core, but now in .net core 3.1 it does not.

So the thing is, after a user registers an account, I send him activation e-mail with activation code created like this:

var activationCode = await userMan.GenerateEmailConfirmationTokenAsync(su);

Next I generate a http url which is formed like that:

http://localhost/api/users/activate?userId=1234&code=abc+860/def==

Now, note that userId is a valid GUID and my activation code is just example, because I really get a long one code. But what's important in this code - it has plus sings(+), slashes and ends with double equal sign.

Now I get the email with valid user guid and still VALID activation code. But, when I click this link and my UsersController starts, something bad happens. This is my activation method in UserController (part of code)

[Route("api/[controller]")]
[ApiController]
public class UsersController : ControllerBase
{
    [HttpGet("activate")]
    [AllowAnonymous]
    public async Task<IActionResult> ActivateAccount([FromQuery] Guid userId, [FromQuery] string code)
    {
        if (userId == Guid.Empty || string.IsNullOrWhiteSpace(code))
            return BadRequest();
    }
}

And the weird thing is that the code parameter is invalid here. The value is just without plus signs. Instead it has spaces. What's wrong?