I have a tour with a 3D model in First Person View mode, it looks fine on desktop and mobile, but when entering VR mode, it is viewed from a very low height, as if the camera were at ground level. Why does this happen?

In most cases, this problem can be the scale of the model, which is assumed to be much larger, which is why the height in VR is not correct.

You should try adjusting the "World Unit equivalent to" parameter within the 3D model settings. The ideal is to click on the "measure in 3D model" button (the one with the cube with the arrow) and there look for an element for which you can have an exact measurement reference and assign it, for example a door that measures 2 meters, or something similar . Once the new value is assigned, republish the tour and test in VR.