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

(N/A) In an atom,electrons are filled according to the $(n+l)$ rule in order of increasing energy.
For $3d$: $n=3, l=2$,so $(n+l) = 3+2 = 5$.
For $4s$: $n=4, l=0$,so $(n+l) = 4+0 = 4$.
Since the $(n+l)$ value for $4s$ is lower than $3d$,the $4s$ orbital has lower energy and is filled first.
During ionization,electrons are removed from the orbital with the highest principal quantum number $(n)$ first. Since $4s$ has $n=4$ and $3d$ has $n=3$,the $4s$ electrons are further from the nucleus and are removed first.