Have you checked for bugs or maybe your tree has anthracnose (fungus)? Some Mango trees are susceptible. Check the back of the leaves to see if there's scale or bugs. Possibly too much water could also cause this...check the roots if you're able to do it carefully to see if the bottom of the pot is very wet but the top part is dry...Good luck! Hopefully, someone else might offer other ideas or advise...

alright thanks. it looks like theres a tiny bit of scaled leaves.