Ive noticed that buildkite pipeline upload translates unquoted yes to boolean true. To test, upload pipeline with a field, say an env mapping as ENVVAR: yes.
I can see this is happening because i print the pipeline before upload, and then i compare with the piepline thats displayed in the timeline tab.
This is actually expected behaviour. When unquoted, YAML treats yes as a boolean because it is considered a YAML keyword. If you enclose yes in quotes, it becomes a string.