>I guess it's pre-negotiated to 19V or something

This kind of defect is extremely rare since this would basically be a USB killer frying every other device you plug it in to. 99% of the time it's the device is missing some resistors on the cc pins which signal to the charger to send 5v. Since usb-c it defaults to 0v until you request something. But USB-A has no CC pins so it just puts 5v out at all times.