For over thirty years I have worked on the assumption that the working voltage of a cap is a maximum and that you could always go up but never down.
As things do change, I decided to do some research into what the manufacturers say, to make sure I am still doing the right thing.
I checked the websites of four manufacturers, Elna, Rubycon, NIC and Teapo. Reading through the cautionary notes from all of these, there is nothing about the working voltage except to say do not exceed it.
Brownout, you say this is well documented. Can you point me in the right direction to find this documentation?