Quality (from Maven's chisel of Scarab) lost after corruption

I lost the quality (20%) of a Glacier Map (and other maps) after corrupting it with a Vaal Orb. Is it a bug or a feature ? Is it specific to the new chisels or can it happen with regular chisels ? I've been playing for years and I nver lost quality when corrupting a map.

Edit : happened again on a 2nd map.
Edit : Happened 3 times in 3 maps.
Edit : Happened 4 times in 4 maps.