That is not correct. IRCC has a clear guidelines that one can work more or less than 15 hours per week to make up 1560 hours. Read below-
"For part-time work, you can work more or less than 15 hours/week as long as it adds up to 1,560 hours."
https://www.canada.ca/en/immigration-refugees-citizenship/services/immigrate-canada/express-entry/eligibility/federal-skilled-workers.html
I have worked 20 hours per week for 40 months(3 years 4 months) and Casee Analyst counted them full time equivalent of 27 months(2 years 3 months). I have got the information from my gcms notes.
@DelPiero07 has correctly advised here.
My work experience letter clearly mentioned I have worked 20 hours per week, did your letter clearly mention the weekly hour? Or it just say Part Time and you have submitted pay slip to show your hours? IRCC officers will not count your hours from pay slips. Your experiebce Letter must confirm the number of weekly hours otherwise they will count exactly 15 hours as part time.