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.
qnoid
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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.