Spring Webflux model in @GetMapping is empty

This appears to be a regression (works with Spring Boot 2.3.0 and fails with 2.3.1). I think it's more likely to be a Webflux issue, but not really sure TBH. Consider this controller:

@Controller
public class SampleController {

    @GetMapping("/")
    public String home(Map<String, Object> model) {
        model.put("time", new Date());
        model.put("message", "Hello");
        return "index";
    }
}

The home() method is not called (I observe in a debugger) before the AbstractView is rendered, so the model is empty. Sample: https://github.com/scratches/SPR-25313.

UPDATE: it also works with Spring Boot 2.4.0-SNAPSHOT. So something is stuck in the middle there.

Comment From: sdeleuze

I made various tests: - It works with Boot 2.3.0.RELEASE and Framework 5.2.7.RELEASE - Boot master and 2.3.x branches fixes the issue - The issue does not happen if you rename the index view to something else or is you use a non-root path

It looks like a duplicate of https://github.com/spring-projects/spring-boot/issues/21909, so I close this issue which is already fixed on Boot side.