Cartography Scarab of Corruption + Vaal Vessel doesn't give 8-mod corrupted maps

Using Cartography Scarab of Corruption says "Non-Unique Maps found in Area are Corrupted with 8 Modifiers" but maps that drop from Vaal Vessels drop corrupted with fewer than 8 mods.

I see that this bug predates the scarab, with multiple bug reports about this occurring with the 8-mod sextant.

Bug report 343,676,613