Thanks for the information. I read through that link but cannot figure out if this is a bug or intentional?
Intentional, the mining client should modify their client to remove the flags as it isn't in use anyways.
Do you know if 0.19.1 can be successfully dropped into an existing .bitcoin profile as to not have to download the whole blockchain again?
There isn't any (only the peers.dat will be discarded, if I'm not wrong) compatibility issues, if that is what you mean.