I have found a solution!
It seems brave stores the keys/credentials in the KDEwallet which is started on brave launch, however to me, the reason for the issue seemed to be brave browser launching before the wallet was unlocked and therefore brave would not have permission to read any of the keys/credentials.
My solution was as simply as installing “kwallet-pam” to have the wallet unlock on system start (which is enough as long as your user password is the same as your kde wallet password) and then I installed “kwalletmanager” which has to be launched from command line with “kwalletmanager5” and then I simply set my password to nothing. This is less secure but my arch installation is on an encrypted volume so I don’t worry about it too much. Now my wallet is unlocked on system startup which means brave always has access to the sync keys and I am therefore no longer being throw out of my sync chain.