That was indeed the issue. The "The time zone used to create and parse date representations. When unspecified, GMT is used." is true for the ISO8601DateFormatter so I must have mixed the two in my head while working on this.

Thank you eskimo! Much love. Personally, my main motivation in understanding why it fails is to be able to confidently say, here is the fix. Having said that, I hear what you are saying. When using fix-format date, you really really should use en_US_POSIX. I'll happily take that. The wording of the TN isn't as strong/clear which lead me to doubt.