MEghan lied about the prince title. Harry knows the rule but he’s so whipped that he crawled up behind MEghan’s lie.
Traditionally, the prince title would have gone as far out as William’s first born son but Elizabeth granted the rest of his kids to be princes and princesses. She didn’t do the same for Harry’s kids as they’re obviously farther down the line of succession. Of course, the msm and Americans use it and the princess title willy nilly as they wish. Titles make for great headline$.
https://www.express.co.uk/news/royal/1340568/Prince-Louis-title-name-Queen-Elizabeth-II-royal-titles
Charles could grant it when he becomes king.