Checkbox not working on modal.

I have tried this code and it seems to be working fine, so I cannot reproduce your issue:

 <mdb-btn rounded color="default" @click.native="login = true"
      >launch login modal <mdb-icon icon="eye" class="ml-1"
    /></mdb-btn>
    <mdb-modal :show="login" @close="login = false">
      <mdb-modal-header class="text-center">
        <mdb-modal-title tag="h4" bold class="w-100">Sign in</mdb-modal-title>
      </mdb-modal-header>
      <mdb-modal-body class="mx-3 grey-text">
        <mdb-input
          label="Your email"
          icon="envelope"
          type="email"
          class="mb-5"
        />
        <mdb-input label="Your password" icon="lock" type="password" />

        <div class="custom-control custom-checkbox">
          <input
            type="checkbox"
            class="custom-control-input"
            id="defaultUnchecked"
            checked
          />
          <label class="custom-control-label" for="defaultUnchecked"
            >Default unchecked</label
          >
        </div>
      </mdb-modal-body>
      <mdb-modal-footer center>
        <mdb-btn @click.native="login = false">Login</mdb-btn>
      </mdb-modal-footer>
    </mdb-modal>

Are you getting any errors in the console? Kind regards, Magdalena