hey lee

i am by no means an expert, but i have managed to keep my fukien tea relatively happy. have you root pruned recently? upper foliage going brown or looking generally unhealthy can mean that there is an imbalance between the root structure and the foliage. bonsai is all about the balance, so if it starts to look unhealthy at the top (especially the top) it tends to suggest the roots arent extensive enough to support the foliage. the leaves browning and dropping off is a natural process of survival - if it is the case of an imbalance - like i say, im not an expert!

it would be useful to know how long you have had it, how you water, what the light is like and any other info you think might help. for now, be careful with the watering - overwatering could make the leaves thick and rubbery like you described.