Issue with ShowIncreasingSubsets Animation: Fill Opacity Change

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## Description of bug / unexpected behavior

I have encountered an issue with the ShowIncreasingSubsets animation in Manim, where the fill opacity of the objects appears to change unexpectedly during the animation.

## Expected behavior

The fill opacity of the object should remain nimation, matching the initial specified value.

## How to reproduce the issue

Code for reproducing the problem

```py
class Test(Scene):
def construct(self):
integers=VGroup(*[Integer( 0 ) for i in range(0,20)]).arrange_in_grid(cols=5)

rects=VGroup(*[
SurroundingRectangle(integers[i ]).set_fill(YELLOW,0.2) for i in range(0,20)
])

self.add(integers)
self.play(FadeIn(rects))

self.clear()

self.add(integers)
self.play(ShowIncreasingSubsets(rects)
```

## Additional media files

Images/GIFs

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## 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 reproducer and compare the initial fill opacity with the result of ShowIncreasingSubsets after FadeIn. Trace ShowIncreasingSubsets and the rectangle opacity handling; done means the rectangles retain the specified 0.2 fill opacity throughout the animation, with a regression test covering this case.

Written by the indexing model from the issue text.