Format of `services.ports.http_options.response.headers` in `fly.toml`

Today I tried adding various HTTP response headers to a Fly app via setting the services.ports.http_options.response.headers option as described here.

Instead of using a TOML inline table, I defined it as follows for better readability:

port = 443
handlers = ["tls", "http"]

Content-Security-Policy = "default-src 'self'; base-uri 'self'; form-action 'none'; frame-ancestors 'none'"
Referrer-Policy = "strict-origin-when-cross-origin"
Strict-Transport-Security = "max-age=63072000; includeSubDomains"
X-Content-Type-Options = "nosniff"
X-Frame-Options = "DENY"

The above works fine.

But when I define the Content-Security-Policy key as a multi-line literal string instead of a normal string (again, for better readability), …

Content-Security-Policy = '''
  default-src 'self';
  base-uri 'self';
  form-action 'none';
  frame-ancestors 'none

…no CSP header is returned anymore after deployment, so there must be some Fly-internal parsing error / incompatibility somewhere.

Defining HTTP response headers with exactly the same syntax (multi-line literal strings that contain semantically irrelevant whitespace) works fine with Netlify (with the netlify.toml config file to be precise).

Why does Fly fail to process such strings?