Synapse is currently trying to put the wrong thing out of the email config for password reset templates.

What it's actually called (password_reset_template_success_html):

What it tries to pull out (password_reset_success_template):

It's the same situation for the failure template.

Therefore, if someone tries to set up custom password reset success/failure pages, Synapse won't take them and consider using the defaults.

The only reason I don't think anyone's reported this yet is because they haven't had the desire to change their password reset success/failure pages from the default.