Instance custom parameter WeightClass is being "resized"

I discovered strange values in my weightClass instance custom parameters. It turns out, when I changed from 2048 to 1000 units per em, all weightClass values in my 72 instances where scaled along. These values don’t refer to the EM ;-(
It happened in October, with whatever version I was using at the time. I can reproduce it with 1175. And if it hasn’t been fixed in later updates, the bug must still be there…