As I understand it, the gene that determines male pattern baldness is located on the X chromosome, so it's simple for cis men, who only have one: if there's baldness on one side of a cis man's mother's family, he has a 50% chance of inheriting it, and if there's baldness on both sides of her family, he has a 100% percent chance.
It's a lot more complicated for a trans guy, who inherits an X chromosome from both of his parents. Not only does that make it harder to calculate the odds, but I doubt that there's enough data from trans guys to explain how their two X chromosomes interact when one carries the gene and the other doesn't.
Another factor, which might help explain your perception that it's common for trans men to lose hair, is that men and women tend to have different hairlines: women's are generally more smooth and rounded, while those of men have more of an M shape. When trans men go on T, their hairlines tend to change to that male pattern, receding at the two sides of the forehead. That doesn't necessarily mean that they're going to go bald, however -- it's just part of the general process of masculinization.
Dunno if this helps -- I hope so.
