There is always a default background on TextInputLayout in Android

https://github.com/material-components/material-components-android/issues/102

It seems a material textfield bug.
You can use this code to change background white temporarily.

<com.google.android.material.textfield.TextInputLayout
   app:boxBackgroundColor="@android:color/transparent" 
   android:background="@android:color/transparent">
   <com.google.android.material.textfield.TextInputEditText/>
</com.google.android.material.textfield.TextInputLayout>

One way to remove the background color is to adjust the background mode of TextInputLayout.

app:boxBackgroundMode="none"

Worked for me.

Example XML Code:

<com.google.android.material.textfield.TextInputLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:boxBackgroundMode="none"
            app:errorEnabled="true"
            app:hintEnabled="false"
            app:layout_constraintTop_toTopOf="parent">

            <com.google.android.material.textfield.TextInputEditText
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:gravity="top"
                android:hint="@string/comment_placeholder"
                android:inputType="textCapSentences|textMultiLine"
                android:maxLength="500" />

        </com.google.android.material.textfield.TextInputLayout>

Method A

(completely transparent background)

<com.google.android.material.textfield.TextInputLayout
    app:boxBackgroundMode="filled"
    app:boxBackgroundColor="@null"
..>

<com.google.android.material.textfield.TextInputEditText
    android:backgroundTint="@android:color/transparent"
..>

Method B

(solid background colour)

<com.google.android.material.textfield.TextInputLayout
    app:boxBackgroundMode="filled"
    app:boxBackgroundColor="?android:attr/colorBackground"
..>

*For my case, I needed it to be fully transparent as the screen background is not a solid colour. If that's the case for you, go with Method A