Do you have a source you can provide?
I should have said, "All my research ...". I was curious during beta after reading:
Arcane Brilliance: Mists of Pandaria mage glyph guide
and did several minutes of testing. From
Arcane Missiles | Lhiveraâ€™s Library
we can see a 30% proc rate of Arcane Missiles. Casting 180 AB with no other spells should yield 54 procs. That's what I got +/- 2 with and without the glyph.
It didn't occur to me to cast Slow by itself and see if it procced Arcane Missiles, as Slow is not, per se, a harmful spell. Is that what happened for you?