Rails nested form 3rd level not saving data

I have a user who can have many reservations and each reservation can have many tables. It saves the user and the reservations, but not the tables. Tables can have special requirements, like wheelchair accessible or kids high chair etc.

Models

User:

class User < ApplicationRecord
  has_many :reservations, dependent: :destroy
  has_many :tables, through: :reservations
  accepts_nested_attributes_for :reservations, reject_if: :all_blank, allow_destroy: true
  accepts_nested_attributes_for :tables, reject_if: :all_blank, allow_destroy: true
end

Reservations:

class Reservation < ApplicationRecord
  belongs_to :user
  has_many :tables
  accepts_nested_attributes_for :tables, reject_if: :all_blank, allow_destroy: true
end

Tables:

class Table < ApplicationRecord
  belongs_to :reservation
end

The form structure looks like this:

Views

user/_form.html.slim

= simple_form_for(@user) do |f|
   .row
     .col-6
       = f.input :last_name, required: true
   .row
     .col-6
       = f.input :first_name
     .col-6
       = f.input :email

... some more here

    = f.simple_fields_for :reservations do |res|
       = render 'reservations/reservations_fields', f: res
       .links
         = link_to_add_association 'add reservation', f, :reservations, partial: 'reservations/reservations_fields'

reservations/_reservations_fields.html.slim:

.form-inputs
  .row
    .col-6
      = f.input :reservation_datetime, as: :datetime, minute_step: 15

... some more here

  .row
    = f.simple_fields_for :tables do |table|
      .col-12
        = render 'tables/form', f: table

tables/_form.html.slim:

.nested-fields
  .form-inputs
    .row
      .col-6
        = f.input :wheelchair
... etc

Controller

users_controller.rb

  def user_params
    params.require(:user).permit(:first_name, :last_name, :phone_number, :email, :company, :comments,
           reservations_attributes: [:id, :reservation_datetime, :important, :head_count, :comments,
              tables_attributes: [:id, :wheelchair, :children, :comments]])
  end

Why are the table attributes not saving? I cannot find the mistake.

Edit: I want to add that it saves the association, but NOT the attributes. Thank you

Edit 2: The params are sent to the UserController (I checked with byebug), but they don't get saved.

byebug:

(byebug) user_params
<ActionController::Parameters {[...], "comments"=>"", "reservations_attributes"=><ActionController::Parameters {"0"=><ActionController::Parameters { [...], "tables_attributes"=><ActionController::Parameters {"0"=><ActionController::Parameters {"wheelchair"=>"0", "children"=>"2", "comments"=>""} permitted: true>} permitted: true>} permitted: true>} permitted: true>} permitted: true>

But the SQL statement is

  ↳ app/controllers/users_controller.rb:42:in `create'
  Tables Create (1.0ms)  INSERT INTO "tables" ("reservation_id") VALUES ($1) RETURNING "id"  [["reservation_id", 12]]
  ↳ app/controllers/users_controller.rb:42:in `create'
   (2.0ms)  COMMIT

Alright, the problem wasn't because it was third-level nested. The real problem was that I used attr_accessor in my tables model. When I removed this, it suddenly worked. Don't know why though, will open another question regarding this.