I can say that with V4.0 firmware in my X-T5 I continue to randomly get photos where nothing is in focus, despite the green focus confirmation appearing. This occurs with single point and single AF selected. It happens with well regarded lenses such as the XF 18mm F1.4 and the Viltrox AF 13mm F1.4.

That looks like oil or some other smear/debris on the sensor.

If you have a lens attached it's more likely the lens. The lenses with electromagnetic focusing have a freely moving focus assembly until the camera is turned on. That is normal. If it's in the body there is a problem.

I can say that v4 firmware works fine in my x-t5.

Have you seen it on other pictures since?