Silent failure on request validation

I working on the "next steps" to Laravel 8 From Scratch- specifically step 2)Update the "Edit Post" page in the admin section to allow for changing the author of a post.

I am trying to better understand why I'm not getting any feedback on the page with dd() after I call the validatePost() method within the update() method in AdminPostController.php. I think I'm missing an understanding of the validatePost() method that is called within update(), but I don't know.

I saw there was an issue with validations on large requests failing silently, but the solution for those was to change SESSION_DRIVER in .env to file from cookie- however, mine is already set to file. Also in session.php I have 'driver' => env('SESSION_DRIVER', 'file'), for what it's worth.

Here's the code within AdminPostController.php:

public function update(Post $post)
    {
        // This works- data from $post is displayed when I click on update on the form
        // dd($post);

        $attributes = $this->validatePost($post);

        // These don't work
        // dd($attributes);
        // dd('test');

        if ($attributes['thumbnail'] ?? false) {
            $attributes['thumbnail'] = request()->file('thumbnail')->store('thumbnails');
        }

        if (is_null($post->published_at) && $attributes['is_published'] == 1) {
            $attributes['published_at'] = now();
        }

        $post->update($attributes);

        return back()->with('success', 'Post Updated!');
    }

protected function validatePost(?Post $post = null): array
    {
        $post ??= new Post();

        return request()->validate([
            'title' => 'required',
            'thumbnail' => $post->exists ? ['image'] : ['required', 'image'],
            'slug' => ['required', Rule::unique('posts', 'slug')->ignore($post)],
            'excerpt' => 'required',
            'body' => 'required',
            'category_id' => ['required', Rule::exists('categories', 'id')],
            'is_published' => 'required'
        ]);
    }

And here is the code for the form in edit.blade.php (I'm not sure how to get the syntax highlighting to work, sorry):

<x-layout>
    <x-setting :heading="'Edit Post: ' . $post->title">
        <form method="POST" action="/admin/posts/{{ $post->id }}" enctype="multipart/form-data"/>
            @csrf
            @method('PATCH')

            <x-form.input name="title" :value="old('title', $post->title)" />
            <x-form.input name="slug" :value="old('slug', $post->slug)"/>


            <div class="flex mt-6">
                <div class="flex-1">
                    <x-form.input name="thumbnail" type="file" :value="old('thumbnail', $post->thumbnail)"/>
                </div>

                <img src="{{ asset('storage/' . $post->thumbnail) }}" alt="" class="rounded-xl ml-6" width="100">
            </div>

            <x-form.textarea name="excerpt">{{ old('excerpt', $post->excerpt) }}</x-form.textarea>
            <x-form.textarea name="body">{{ old('body', $post->body) }}</x-form.textarea>

            <x-form.field>
                <x-form.label name="category" />

                <select name="category_id" id="category_id">
                    @foreach (\App\Models\Category::all() as $category)
                        <option
                            value="{{ $category->id }}"
                            {{ old('category_id', $post->category_id) == $category->id ? 'selected' : ''  }}
                        >{{ ucwords($category->name) }}</option>
                    @endforeach
                </select>

                <x-form.error name="category" />
            </x-form.field>

            @if ((bool)!$post->is_published)
                <x-form.field>
                    <x-form.label name="Do you want to publish your post?" />
                    <select name="is_published" id="is_published">
                        <option value=1 >Yes</option>
                        <option value=0 selected>No, keep it as a draft</option>
                    </select>
                </x-form.field>
            @endif

            <x-form.button>Update</x-form.button>
        </form>
    </x-setting>
</x-layout>