[Bug Report][3.6.12] v-btn-toggle in variant="plain" does not show default value as selected

[preiaen]

Environment

Vuetify Version: 3.6.12
Vue Version: 3.4.27
Browsers: Firefox 128.0
OS: Windows 10

Steps to reproduce

In case the play.vuetifyjs.com link doesn't work the following script shows the issue:

<template>
  <v-container>
    <v-row>
      <v-col class="py-2" cols="12" sm="6">
        <p>Mandatory</p>
        <v-btn-toggle v-model="toggle_one">
          <v-btn value="eins">
            <v-icon>mdi-format-align-left</v-icon>
          </v-btn>

          <v-btn value="zwei">
            <v-icon>mdi-format-align-center</v-icon>
          </v-btn>

          <v-btn value="drei">
            <v-icon>mdi-format-align-right</v-icon>
          </v-btn>

          <v-btn value="vier">
            <v-icon>mdi-format-align-justify</v-icon>
          </v-btn>
        </v-btn-toggle>
      </v-col>

      <v-col class="py-2" cols="12" sm="6">
        <p>Mandatory</p>
        <v-btn-toggle v-model="toggle_one" variant="plain">
          <v-btn value="eins">
            <v-icon>mdi-format-align-left</v-icon>
          </v-btn>

          <v-btn value="zwei">
            <v-icon>mdi-format-align-center</v-icon>
          </v-btn>

          <v-btn value="drei">
            <v-icon>mdi-format-align-right</v-icon>
          </v-btn>

          <v-btn value="vier">
            <v-icon>mdi-format-align-justify</v-icon>
          </v-btn>
        </v-btn-toggle>
      </v-col>
    </v-row>
  </v-container>
</template>

<script setup>
  import { ref } from 'vue'

  const toggle_one = ref('vier')
</script>

Expected Behavior

The expected behavior would be, that the fourth element in plain variant is bold or in a different color, like it is, when one actualy clicks on it.

Actual Behavior

The regular variant shows the fourth element as selected while the plain variant does not.

Reproduction Link

https://play.vuetifyjs.com/#...