Rules are converted to JavaScript. Unfortunately, JavaScript sees a "0" as false. Therefore, if you have a conditional mandatory rule that is looking for a value of "0" in a field, the boolean that will be returned will Always be false.
For example:
Field A can be a value of "0" or "1"
Field B is mandatory if Field A = "0"
The expression Field A = "0" will always return false
You may need to reconsider how you set up either the values or the rule to avoid using "0" in the simple expression.