While filling up of electrons in the atomic orbitals,the $4s$ orbital is filled before the $3d$ orbital but reverse happens during the ionisation of the atom. Explain why?

(N/A) The filling of orbitals is governed by the $(n+l)$ rule,also known as the $Aufbau$ principle. For the $4s$ orbital,$(n+l) = 4+0 = 4$. For the $3d$ orbital,$(n+l) = 3+2 = 5$. Since $4 < 5$,the $4s$ orbital is filled first.
During ionization,electrons are removed from the orbital with the highest principal quantum number $(n)$. In the case of transition metals,the $4s$ orbital has $n=4$,while the $3d$ orbital has $n=3$. Therefore,the $4s$ electrons are at a greater average distance from the nucleus and are removed first.