Resetting the opacity of NumberLine numbers doesn't animate smoothly

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## Description of bug / unexpected behavior

Create a number line with numbers. Animate a change in the opacity of the numbers, accessed with ```NumberLine.numbers```. The opacity changes abruptly rather than smoothly.

Smooth opacity change can be obtained by putting the numbers in a VGroup, but then the axis disappears.

## Expected behavior

The opacity should change smoothly without removing the axis.

## How to reproduce the issue

Code for reproducing the problem

```py
from manim import *

class numberline(Scene):
def construct(self):

abrupt_opacity = Tex("Opacity changes abruptly:").shift(UP)
number_line = NumberLine(include_numbers = True)
self.add(abrupt_opacity, number_line)
self.wait(1)
self.play(
number_line.numbers.animate.set_fill(opacity = .2)
)
self.wait(1)
self.remove(abrupt_opacity, number_line)

self.wait(1)

axis_disappers = Tex("Axis disappears:").shift(UP)
number_line = NumberLine(include_numbers = True)
self.add(axis_disappers, number_line)
self.wait(1)
self.play(
VGroup(*number_line.numbers).animate.set_fill(opacity = .2)
)
self.wait(1)
```

## Additional media files

Images/GIFs

https://user-images.githubusercontent.com/4729700/213765636-60e75df1-f128-42eb-86f1-4461c3a253eb.mp4

## Logs
Terminal output

```
PASTE HERE OR PROVIDE LINK TO https://pastebin.com/ OR SIMILAR
```

## System specifications

System Details

- OS (with version, e.g., Windows 10 v2004 or macOS 10.15 (Catalina)):
- RAM:
- Python version (`python/py/python3 --version`):
- Installed modules (provide output from `pip list`):
```
PASTE HERE
```

LaTeX details

+ LaTeX distribution (e.g. TeX Live 2020):
+ Installed LaTeX packages:

FFMPEG

Output of `ffmpeg -version`:

```
PASTE HERE
```

## Additional comments

Start by running the provided reproduction and inspecting how NumberLine.numbers are represented and animated. Compare direct animation of number_line.numbers with animation of VGroup(*number_line.numbers); done means opacity changes smoothly while the axis remains visible.

Written by the indexing model from the issue text.