Validation of "+" symbol always fails in jUnit

I have this simple controller in spring boot with parameter validation:

@ApiOperation(value = "Test")
@ApiResponses(value = {
        @ApiResponse(code = 200, message = "OK"),
        @ApiResponse(code = 400, message = "Bad request")
})
@PutMapping(value = "/test")
@ResponseStatus(value = HttpStatus.OK)
public void test(
        @ApiParam(value = "Plus", required = true)
        @Pattern(regexp = "^\\+$")
        @RequestParam(value = "plus") String plus) {
    System.out.println("Validation passed: " + plus);
}

As you can see Plus field only accepts "+" symbol. When I try to run it and fire request on localhost swagger it works. Validation passes, I get HTTP 200 status.

Generated swagger file looks good, validation is there: pattern: "^\\+$"

If I try to run a jUnit test, validation fails, I get 400 Bad Request. In test I use controller that is generated from swagger file:

@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.DEFINED_PORT)
@ActiveProfiles("test")
@DirtiesContext
public class CeCommissioningBffControllerTest {
    
    @MockBean
    private controllerApi api; //This is generated class from swagger file

    @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
    }

    @Test
    public void test(){
        api.test("+");
    }
}

Why does it fail in jUnit? I tried to replace '+' with "$" symbol everywhere and amazingly it works then.