Interesting scenario...
I suppose their argument makes sense. In theory they reversed that charge so you were not entitled to the $50.
Per the terms
You can earn a $50 statement credit per qualifying Hyatt purchase with your World of Hyatt Business Credit Card up to 2 times each anniversary year (for a total of $100).
I guess their argument would be that its a statement credit, so it can only be used to offset charges...if there are no charges, its lost. Their system might not usually reverse them, but they might be able to code it somehow as statement credits not being refundable.