Looking at the players history I think it may have something to do with the 'regardless of loan spells away from the club' which might be causing the issue, which to me means to disregard any loans. His history shows he was on loan at the club during his 17th and 18th birthdays. Then he had signed permanently for his 19th, was sent out on loan for his 20th (which is fine), and turned 21 during the his 3rd season of having a permanent contract with the club (born in October 2000). So he signed permanently in July 2019 (aged 18) and so has only trained at the club for 2 years and 3 months by the time he reached 21 in October 2021. I think it's those two years on loan in 17/18 and 18/19 that don't count towards the home grown at club rule.