Repro steps:
-Create a business rule on an Entity (For e.g. Account)
-Create a condition
-Add an Action (Set Field Value)
-Field - (Decimal number field) Type - Formula
-Field - (Whole number field) Operator (*) Type (Value) Value = (0.8)
-Save and activate the business rule
-The value 0.8 returns back to Zero

Since the value returns back to Zero therefore, we cannot calculate actual amount and this result in incorrect data amount resulting in loss when calculations are done in thousands.