Manim's unexpected colouring behaviour under the radical sign

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## Description of bug / unexpected behavior
Hi,

I want to color the two's in purple and the three's in yellow by running these code on manim:

**first code**
```py
import numpy as np

class vectorTry(Scene):
def construct(self):
val_a = 3
val_b = 2
color_a = "#f1f514"
color_b = "#cf0492"
tex_a = Tex(str(val_a).format(int), color=color_a).move_to([-1,2,0]).scale(2)
tex_b = Tex(str(val_b).format(int), color=color_b).move_to([1,2,0]).scale(2)

self.play(FadeIn(tex_a, tex_b))
self.wait()

#using \over because \frac fails here
form = MathTex(r'\hat{u}= { 3 \hat{i} + 2\hat{j} \over{ \sqrt{ 2 ^{2} + 3 ^{2} } } }',
substrings_to_isolate=["2", "3"]).scale(1.25)

idx_a = [int(i) for i, character in enumerate(form) if character.get_tex_string() == tex_a[0].get_tex_string()]
idx_b = [int(i) for i, character in enumerate(form) if character.get_tex_string() == tex_b[0].get_tex_string()]

for i in range(len(form)):
print(form[i])

print(idx_a)
print(idx_b)

get_pos_a = form[idx_a[0]].get_center()
get_pos_b = form[idx_b[0]].get_center()
a1_copy = []
b1_copy = []

#here I force colouring of two's and three's on the MathTex object
for i in range(len(idx_a)):
a1_copy += form[idx_a[i]].set_color(color_a)
for i in range(len(idx_b)):
b1_copy += form[idx_b[i]].set_color(color_b)

self.play(FadeIn(form))
self.play(tex_a.animate.move_to(get_pos_a).match_height(form[idx_a[0]]))
self.play(tex_b.animate.move_to(get_pos_b).match_height(form[idx_b[0]]))
self.wait(1)
```

or

**second code**
```py
import numpy as np

class vectorTry(Scene):
def construct(self):
val_a = 3
val_b = 2
color_a = "#0470cf"
color_b = "#cf0492"
tex_a = Tex(str(val_a).format(int), color=color_a).move_to([-1,2,0]).scale(2)
tex_b = Tex(str(val_b).format(int), color=color_b).move_to([1,2,0]).scale(2)

self.play(FadeIn(tex_a, tex_b))
self.wait()

#using \over because \frac fails here
form = MathTex(r'\hat{u}= { 3 \hat{i}+ 2\hat{j} \over{ \sqrt{ {2^{2}+3^{2} } } } }',
substrings_to_isolate=["2", "3"]).scale(1.25)

idx_a = np.array([i for i, character in enumerate(form) if character.get_tex_string() == tex_a[0].get_tex_string()],
dtype='int')
idx_b = np.array([i for i, character in enumerate(form) if character.get_tex_string() == tex_b[0].get_tex_string()],
dtype='int')

get_pos_a = form[idx_a[0]].get_center()
get_pos_b = form[idx_b[0]].get_center()

#using the set_color methods of MathTex object
form.set_color_by_tex(str(val_a).format(int), color_a)
b1_copy = form[idx_b[0]].set_color(color_b)

self.play(FadeIn(form))
self.play(tex_a.animate.move_to(get_pos_a).match_height(form[idx_a[0]]))
self.play(tex_b.animate.move_to(get_pos_b).match_height(form[idx_b[0]]))
self.wait(1)
```

Fails to color the two's and three's below the radical correctly.

## Expected behavior
The two's below the radical should have coloured in purple, the three's in yellow.

## Additional Info

Printing the indices of two's and three's seems that manim is aware of the indices of the numbers.

```py
for i in range(len(form)):
print(form[i])

>>> SingleStringMathTex('\\hat{u}= { ')
SingleStringMathTex('3')
SingleStringMathTex(' \\hat{i} + ')
SingleStringMathTex('2')
SingleStringMathTex('\\hat{j} \\over{ \\sqrt{ ')
SingleStringMathTex('2')
SingleStringMathTex(' ^{')
SingleStringMathTex('2')
SingleStringMathTex('} + ')
SingleStringMathTex('3')
SingleStringMathTex(' ^{')
SingleStringMathTex('2')
SingleStringMathTex('} } } }')

print(idx_a)
print(idx_b)

>>> [1, 9]
[3, 5, 7, 11]
```

## Additional media files
Whether I run them in the discord page of manim or their jupyter binder, the result is the same.

**first code output**
https://user-images.githubusercontent.com/47322232/131674737-1ccf5afa-a4f8-49cc-b986-47f1f19ffcc5.mp4

**second code output**
https://user-images.githubusercontent.com/47322232/131674520-c001f054-c73b-404d-b7a4-b77a84382de7.mp4

Reproduce the two provided MathTex examples, then start at the MathTex entry point and the set_color_by_tex and set_color calls used in the report. Done means the occurrences of 2 and 3 below the radical receive the requested colors consistently; the payload names no source files or tests.

Written by the indexing model from the issue text.