ParseException only during unit testing

I'm writing unit tests for Android code using the Robolectric gradle test runner. The code I'm testing happens to be hitting date formatting methods that use the following format:

yyyy-MM-dd'T'HH:mm:ss.SSSZ

We store Unix Time milliseconds as strings in that format, and just before sending it through the formatter to convert it back into a millisecond offset, we replace any instances of "Z" in the string with "+00:00". the call ends up looking like this:

DateFormat format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ");
format.setTimeZone(TimeZone.getTimeZone("UTC"));
format.parse(validDateString.replace("Z", "+00:00));

This operation works fine in production code, but attempting to write unit tests has revealed previously unseen ParseExceptions. I first assumed it was because of a formatting issue with the date string I was injecting, but ParseExceptions are being thrown on strings saved from a date successfully parsed in prod code.

What could possibly be causing this radical difference in behavior?

Things I've tried already:

-Checked date formatting

-This DateFormat is actually a global static variable. I'm aware that they're not thread-safe, but inlining all static references with new instances yields the same results.