Transitions isn't working in css React.js

I have a css property transition set to height 3s. However, it isn't working. I m on react.js

When I try to do it, there is no transition on height.

Short part of code. Jsx:

function Form({showPopup}){
  const [height, setHeight] = useState("400px")
    function proceedWithOtp(message){
      setHeight("300px")
      setForm(<OtpForm message={message}/>)
    }
    const [form, setForm] = useState(<RegisterForm showPopup={showPopup} proceedWithOtp={proceedWithOtp}/>)

    return (
      // This system is used so that height animation can work
      <div className="register-form" style={{
        height: height
      }}>
        {form}
      </div>
    )
}

Css:

.Register .register-form form {
    width: 350px;
    padding: 25px;
    border-radius: 25px;
}

.Register .register-form {
    display: flex;
    justify-content: center;
    align-items: center;
    transition: height 5s;
}

.Register .register-form form * {
    padding: 5px;
}

This is what RegisterForm returns:

        <form onSubmit={handleSubmit}  className="bg-white text-dark email-form">
            <h1 className="text-center font-hagrid">Register</h1>
            <br />
            <div className="mb-3">
    <input type="text" value={name} onChange={e => setName(e.target.value)} required placeholder="Enter your Name" className="form-control"  />   
  </div>

  <div className="mb-3">
    <input type="email"value={email} onChange={e => setEmail(e.target.value)} required placeholder="Enter your email address" className="form-control" />   
    {/* <div id="emailHelp" className="form-text">Your passwords are 100% secure and encrypted with us!</div> */}
  </div>
  <div className="mb-3">
    <input placeholder="Enter your password" required value={password} onChange={e => setPassword(e.target.value)}type="password" className="form-control" />
  </div>
      <ReCAPTCHA
      sitekey={RECAPTCHA_SITE_KEY}
      size="invisible"
      ref={recaptchaRef}
      />
   <button disabled={buttonDisabled} type="submit"  className="btn btn-primary btn-submit text-dark">{buttonContent}</button>
</form>

OtpForm returns:

 <form className="bg-white text-dark otp-form" >
<p className="text-center">{message}</p>
<div className="input_field_box mb-3">
<input ref={ref1} name="1" onChange={otpBoxChange}  type="number" />
<input ref={ref2} name="2" onChange={otpBoxChange}  type="number" disabled />
<input ref={ref3} name="3" onChange={otpBoxChange}  type="number" disabled />
<input ref={ref4} name="4" onChange={otpBoxChange}  type="number" disabled />
</div>
<button disabled={false} type="submit"  className="btn btn-primary btn-submit text-dark">{"Submit"}</button> 
</form>

This is a form for register. The items are centered with a definite width. The height is fixed and I want it to animate. However, for some reason, its not animation with the transition property on css.
How do I fix this now? I'm unable to figure it out.

CSS can't directly animate the height property when it is set with auto (https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_animated_properties).
One thing you could try is setting the height via JavaScript using the scrollHeight property, or try one of the numerous solutions already discussed here.