Form body in a POST ending up as URL params?

Our stack:

• Vue.js frontend using vuetify component lib
• custom python middleware rest api using flask + tornado
• matomo running externally and connected to the frontend using vues plugin system.(https://github.com/AmazingDreams/vue-matomo)

We recently added matamo to our site and very very rarely we've noticed 4 incidents out of thousands of users where the username/password which is submitted via POST request to our middleware is ending up being logged in matomo as https://somesite.com?username=someUser&password=somePassword.

Strangely enough the actual route to login is at somesite.com/login so its weird matamo sees it on the homepage.

Here's the code we use for logging users in:

auth.js

const authenicateUser = async (username, password) => {
const body = { username: username, password: password }
const headers = new Headers()
headers.append('Content-Type', 'application/json')
headers.append('Accept', 'application/json')
try {
  const response = await fetch('https://somesite.com/users/login', {
    method: 'POST',
    ...(body ? { body: JSON.stringify(body) } : {}),
    cache: 'no-store',
    credentials: 'include', // this is to allow cross origin requests to our middleware microservice
    headers: headers
  })
  return response
} catch (error) {
  return false
}
}

login form

<v-form @submit.prevent="submit" @keyup.native.enter="submit" id="check-login-form">
            <v-text-field
              class="input-field"
              label="MS ID"
              v-model="username"
              name="username"
              data-cy="userName"
              prepend-icon="mdi-account"
              type="text"
              color="rgb(232, 119, 34)"
            />
            <div class="password-field">
              <v-text-field
                class="input-field"
                id="password"
                data-cy="userPassword"
                label="Password"
                v-model="password"
                name="password"
                prepend-icon="mdi-lock"
                :type="showPassword ? 'text' : 'password'"
                @click:append="showPassword = !showPassword"
                color="rgb(232, 119, 34)"
              ></v-text-field>
              <div v-if="showPassword" class="icon-container" v-on:click="toggleShowPassword">
                <img src="~assets/Icons/View.svg" class="eye-icon" />
              </div>
              <div v-else class="icon-container" v-on:click="toggleShowPassword">
                <img src="~assets/Icons/ViewHide.svg" class="eye-icon" />
              </div>
            </div>
          </v-form>

submit method

async submit() {
      this.isLoading = true
      const response = await authenticateUser(this.username, this.password)
      this.statusCode = response.status
      this.currentStatusCode = this.statusCode
      if (this.statusCode === 200) {
        this.currentStatusCode = this.statusCode
        this.$router.push('/')
        this.isLoading = false
        this.$matomo.setUserId(this.username)
      } else {
        this.isLoading = false
        this.currentStatusCode = null
        this.showPassword = false
      }
    },
    toggleShowPassword: function() {
      this.showPassword = !this.showPassword
    }
  },

Any ides on why this might be happening?

We ended up resolving this by adding the method="POST" attr to <v-form>. turned out in some rare cases the form would try to submit as GET which resulted in the form params ending up in the URL as URL params.

 <v-form
   method="POST"
   enctype="text/plain"
   @submit.prevent="submit"
   @keyup.native.enter="submit"
   id="check-login-form"
 >...</v-form>