Fixed-in-frame mobject is not transforming correctly in 3D scene

Nobody has claimed this yet.

Here is the code:

```python
class Test(ThreeDScene):
def construct(self):
axes = ThreeDAxes()
self.set_camera_orientation(phi = 75 * DEGREES, theta = 45 * DEGREES)
self.play(Create(axes))
eq1 = MathTex(r'x')
eq1.to_corner(UL)
self.add_fixed_in_frame_mobjects(eq1)
self.play(Create(eq1))
eq2 = MathTex(r'x^2')
eq2.to_corner(UL)
self.play(Transform(eq1, eq2))
```

In my computer, the animation it produced does not transformed formula $x$ into $x^2$. It only shifts the text $x$ downward but does not write the superscript $2$.

https://github.com/ManimCommunity/manim/assets/86477788/df7603bf-d594-404e-9a14-6e6a14bd02a0

Start by reproducing the supplied ThreeDScene example using ThreeDAxes, MathTex, add_fixed_in_frame_mobjects, and Transform. Inspect the fixed-in-frame mobject transformation path and verify that x transforms into x^2, including the superscript, while remaining fixed in the frame.

Written by the indexing model from the issue text.