that's wrong, Ca cannot be re-mobilised (better: retranslocated) which is the reason why a Ca-def results in stunted chlorotic shootgrowth, the necrotic spots appear somewhat later when the leaf becomes dysfunctional. And why you need a steady supply of Ca in water.
Mg is mobile and that's the reason why it's taken from the older leaves. In times of need, plants disassembles lower chlorophyll, attach the minerals to transporter-proteins, and feed these into the phloem where they can travel up or down.
But pH problems (xylem not neutral) can actually "sabotage" the ability to retranslocate nute-ions, then mobile nute defs can also appear mid or top the plant. The xylem's and phloem's pH is apart by +-1.5 - 2.0 and this creates a "pressure" to make it possible.
only in the case of photodestruction from too high irradiance, then - and with a true Mg def, the leaves will show striped chlorosis, unlike N or S def, which will affect the complete leafcolour