You are not actually streaming FLAC though. It is not impossible to stream FLAC on BTv3 if '+HS' is supported, but I almost never see a device that support '+HS' profile.
The fact is, when you send you FLAC file over BT, it goes to the BT chip and it re-compressed the FLAC files with another codec - most commonly SBC, but it can be mp3 or wma up to 320kbps depends on the A2DP profile. Actually even SBC nowadays can do 320kbps as well. There are some misunderstanding that the apt-X used in BT (under A2DP as well) is lossless, which isn't the case. It is called Enhanced apt-X, and still a lossy codec, but it is much less lossy than SBC for sure. The actually lossless version of apt-X is used on pro-audio only as far as I know, but not on BT. While SBC might looks like it have a lot of drawback - a well implemented SBC can actually be good enough to match between mp3 and wma., and well above the listening threshold for compression artifact. Someone actually did some measurement awhile ago (here)
The fact is, when you send you FLAC file over BT, it goes to the BT chip and it re-compressed the FLAC files with another codec - most commonly SBC, but it can be mp3 or wma up to 320kbps depends on the A2DP profile. Actually even SBC nowadays can do 320kbps as well. There are some misunderstanding that the apt-X used in BT (under A2DP as well) is lossless, which isn't the case. It is called Enhanced apt-X, and still a lossy codec, but it is much less lossy than SBC for sure. The actually lossless version of apt-X is used on pro-audio only as far as I know, but not on BT. While SBC might looks like it have a lot of drawback - a well implemented SBC can actually be good enough to match between mp3 and wma., and well above the listening threshold for compression artifact. Someone actually did some measurement awhile ago (here)