Added to the BMM 5th printing (just released, though the errata has been out for a few months):
⑤ Armor (p. 114)
After “Critical Hits”, insert the following new paragraph:
PSRs: For armor types that can absorb two points of damage per armor circle, only full armor circles removed count when determining if the ’Mech has to make the PSR required from suffering 20+ points of damage.
So no PSR, because 20 armour circles were not removed. This is already partially reflected in the most recent TO printing, for hardened armour ("Each full armor bubble lost only counts as one point of damage for Piloting Skill Roll purposes"), but Laser-Reflective will be updated to clarify this as well in the next print.