Route breaks Single Entry
Craft automatically includes the entry variable if the slug matches the url:


  If the URI matches an entry’s or category’s URI, the
  section’s/category group’s template will get loaded, and the matched
  element will be made available to the template via a pre-populated
  entry or category variable.


In your case contact-us gets matched automatically, but contact-us/thank-you does not. This means you have to set entry yourself.

Just add this to the top of your template:

{% if entry is not defined %}
    {% set entry = craft.entries.section('contact-us').first() %}
    {# Fetch single which has 'contact-us' as a handle #}
{% endif %}