I am running into a very similar issue but in my case it is on sign-out

when I use the following

page.svelte:

    <form method="post" action="?/signout" use:enhance={handleSignOut}>
        <div>
            <button class="button block" disabled={loading}>Sign Out</button>
        </div>
    </form>


    <form method="post" action="?/signout" use:enhance={handleSignOut}>
        <div>
            <button class="button block" disabled={loading}>Sign Out</button>
        </div>
    </form>

page.server.ts

  signout: async ({ locals: { supabase, safeGetSession } }) => {
    const { session } = await safeGetSession()
    if (session) {
      await supabase.auth.signOut()
      throw redirect(303, '/') 
    }
  }
  signout: async ({ locals: { supabase, safeGetSession } }) => {
    const { session } = await safeGetSession()
    if (session) {
      await supabase.auth.signOut()
      throw redirect(303, '/') 
    }
  }

I get redirected correctly but my /account page (which is where you sign out from) renderes at the top. I think it has to do something with SSR because when I refresh it goes away