@secretchimp - did you ever find an answer to this question? I'm in a situation where I'd like to specify sandbox for an ad-hoc build that points at a test environment.
CloudosaurusRex
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After more investigation I discovered the root cause here, and it was due to my own misunderstanding.
One of the number formatters had numberStyle set to decimal and another was set to currency. I didn't realise that a different grouping separator was used for currency (currencyGroupingSeparator) vs. other number styles (groupingSeparator), so that's on me for not comprehensively reading the docs!
By ensuring the same grouping separator was applied consistently, this problem went away.
Posting this reply in case anyone else encounters a similar situation and is confused!