According to research, it isn’t uncommon for sexual attraction or orientation to change after transitioning from one gender to another.
Most of these changes include shifts from exclusive attraction to one gender pre-transition toward some level of bisexuality post-transition…But some people claim almost complete reversal of their sexual orientation. For example, 13% of the trans women in the 2005 study switched from exclusive or primary attraction to women to exclusive or primary attraction to men (there were no such drastic changes among those who were initially attracted to men). Of the trans men in the 2013 study who were initially attracted to either men or women, 6-7% experienced a complete reversal.
It’s not clear why these changes occur, but there could definitely be more than one correct answer. Sexual attraction and orientation is not necessarily something that is fixed at birth. There is plenty of evidence to suggest that it is fluid.