Redirect failure on confirmation page

I finally found out what was causing this. We had default_page_base_path: set to "form". The correct value would be "/form". On the module's configuration page, this is labelled "Default base path for webform URLs".

To fix this I needed to delete the broken aliases, update this value for the webform module and finally resave any related webforms.
It looks like webform_update_8606() tried to address this, but only in one isolation case.

Hopefully this helps someone else struggling with a similar situation.