Why the same button animation cannot use inside form?

I tried to add an animation for my form submit button on my Todo list.

Unfortunately, the same button animation works outside form, but inside form only can hover the text. I have no idea where the problem is?

import React from "react";

class TodoForm extends React.Component {
    render() {
        return (
            <div>
                <div className={"container"}>
                    <form action="">
                        <h1>Todo List</h1>
                        <input type="text" placeholder={""}/>
                        <button className="custom-btn btn">Submit</button>
                    </form>
                    <button className="custom-btn btn">Submit</button>
                </div>
            </div>
        );
    }
}

export default TodoForm;

.container {
    display: flex;
    flex-direction: row;
    /*align-items: center;*/
    justify-content: center;
    margin-top: 5%;
}

form {
    height: 500px;
    width: 400px;
    background-color: #f4f7fc;
    border: 2px solid;
    border-radius: 10px;
    box-shadow:8px 8px 5px;
    text-align: center;
}

h1 {
    font-family: 'Orbitron', sans-serif;
    font-weight: 900;
}

input[type='text'] {
    border: 1px solid #ddd;
    height: 6%;
    min-width: 60%;
    transition: all ease-in 0.25s;
}

.custom-btn {
    margin-left: 5px;
    width: 25%;
    height: 37px;
    border-radius: 5px;
    background: transparent;
    cursor: pointer;
    transition: all 0.3s ease;
    position: relative;
    display: inline-block;
    outline: none;
}

.custom-btn, input[type='text'] {
    box-shadow: 1px 1px 1px 0px rgba(255,255,255,.5),
    7px 7px 20px 0px rgba(0,0,0,.1),
    4px 4px 5px 0px rgba(0,0,0,.1);
}

.btn {
    border: 2px solid #f4f7fc;
    color: #f59cb1;
    font-family: 'Orbitron', sans-serif;
    font-weight: 900;

}
.btn:after {
    position: absolute;
    content: "";
    width: 0;
    height: 100%;
    top: 0;
    left: 0;
    direction: rtl;
    z-index: -1;
    box-shadow:
            -7px -7px 20px 0px #fff9,
            -4px -4px 5px 0px #fff9,
            7px 7px 20px 0px #0002,
            4px 4px 5px 0px #0001;
    transition: all 0.3s ease;
}
.btn:hover {
    color: #FF6F91;
}
.btn:hover:after {
    left: auto;
    right: 0;
    width: 100%;
}
.btn:active {
    top: 2px;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>

Answer

The background color doesn’t let you see the animation, just comment it;

form {
    height: 500px;
    width: 400px;
    /*background-color: #f4f7fc;*/
    border: 2px solid;
    border-radius: 10px;
    box-shadow:8px 8px 5px;
    text-align: center;
}