As I said, I tried multiple devices: MPOW headphones and August BT Speakers with Media Player, Foobar and MusicBee -- all possible combinations of which (with the very same devices, not newly purchased ones) worked perfectly prior to 1803.
The buttons themselves function, as they control the Windows volume and summon Cortana, so it is not those.
Point being, they behave identically for me in 1809 as they did in 1803 -- same bug, not different ones.
EDIT: I know I shouldn't call it a bug since Microsoft would appear to have hijacked the buttons intentionally...