If you're searching the online documents, they may not contain information that was not changed from the previous contract.
Here in Central PA, "Article 50 - Part Time Employees", notes...
"Part time employees are defined as employees who when reporting to work as scheduled shall be guaranteed a minimum of three and one-half (3-1/2) hours. Should any part time employee work beyond the fifth (5th) hour, they shall be paid time and one-half (1-1/2) unless they were previously scheduled to work eight (8) hours in which case they shall be guaranteed eight (8) hours straight time pay. All time worked over eight (8) hours will be paid at time and one-half (1-1/2)."
As others have mentioned, you have to ask for the guarantee, otherwise they treat it as you volunteered to leave early.
Also, unless you are a part time employee and were scheduled to work or "volunteered" for a double shift, you would be paid time and one-half for working more than 5 hours.
Several years ago, management tried to pay straight time for working more than 5 but less than 8 hours - when it benefited the company. But, those of us who grieved this practice were properly reimbursed for all the overtime we worked.