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Following, the amount entered in the price amount field gets truncated when the value is >= 1000.
In _commerce_price_rules_data_ui_element_validate() the amount is passed through number_format(). PHP docs state that
Note: PHP doesn't handle strings like "12,300.2" correctly by default.
Any value >= 1000 ends up being formatted to that which results in everything after the comma being dropped when rounding.