This seems to happen when navigating using Inertia (using the Link component for example) to a route that returns a normal blade view rather than an Inertia page.

I've seen this behavior by coincidence while playing with Inertia in Django when I tried to navigate to the Django admin panel using an Inertia Link component and it popped up on the screen like it did on your screenshot.

When I realized the reason, I tried to simulate the same behavior using Laravel, and sure enough, it did the same thing.