Yes, that is a known issue with Raspbian Buster currently. The built-in headphones device/driver is reporting that there are 8 channels/speakers available, and BASS will initialize it to use all 8 by default, but that's resulting in garbled sound like you say. You can avoid that problem by using the BASS_DEVICE_STEREO flag in your BASS_Init call to limit the output to stereo. In future, BASS could perhaps try to detect the RPi headphones device and automatically limit the output to stereo then.
This issue should only affect using the headphones device directly. Using it via PulseAudio (eg. when you right-click the volume control and choose "AV Jack") shouldn't be affected because that reports a very high channel/speaker count, which BASS ignores by default (unless BASS_DEVICE_SPEAKERS is used). The built-in HDMI output is also unaffected (it can do 8 channel output without being garbled).
