Since version 0.0.12 flac123 uses stderr to communicate with the calling program. This used to be stdout and so it is (and has been) with mpg123 and mpg321. The problem is that errormessages from flac123 are also written to stderr and this may lead to problems.
Fix: revert to using stdout for writing @R, @P etc. messages (i.e. in remote.c and flac123.c)
Error messages can ofcourse keep on using stderr.
Fixed in version 2.1.0 on github