One potential for how this could happen relates to Ansible’s variable precedence. Suppose you were to have these same variables defined in group_vars/development/. Such (unmodified) variables would override your modified variables in group_vars/all/. (Any particular group in group_vars will take precedence over the all group.)
You could search your Trellis files for other conflicting definitions of these vars, potentially even in temp files. Of course, the values in roles/nginx/defaults/main.yml shouldn’t be conflicting because they have the lowest priority.
When working out variable precedence, it is often helpful to temporarily insert a debug task into the relevant task file or context, just to ensure you know what Ansible is using for variable values. Example:
- debug:
msg: |
nginx_fastcgi_buffers is `{{ nginx_fastcgi_buffers }}`
nginx_fastcgi_buffer_size is `{{ nginx_fastcgi_buffer_size }}`