Hmm, that's probably due to the switch from
QOpenGLWidget in Tiled 0.18. This avoided the additional dependency on QtOpenGL and solved at least one bug, mentioned in the changelog as "Fixed OpenGL rendering mode when application is scaled (HiDpi screens)".
But, I didn't realize it could affect the grid like that. It's certainly rendering fine for me on Linux. I'll give it a try on Windows soon, but I'm not sure what I could do about it.
The easiest fix with the new Tiled version would be to turn off hardware accelerated rendering in the Preferences. It's actually off by default since it brings no advantages over CPU rendering in most scenarios.