Does this have to do with it?
ABW (ADSR Bug Workaround): (independent from L/R flag) an option which provides a method to overcome the ADSR bug.
The drum engine handles this option differently from the other engines. The test bit will be set together with gate for ca. 30 mS in order to reset the oscillator and to start the envelope. The waveform generator will be turned off durig this time in order to disable the sound output. After this delay, the waveform will be enabled again, and the drum sound starts immediately without wacky timings.
This approach is also known as "hard-sync", and used in most C64 tunes.
It results into a more deterministic sound output, but the latency makes it unsuitable for live playing. So, this feature can only be used in conjunction with the internal sequencer, or with an external sequencer which allows to compensate the delay.
Note also, that a part of the ADSR attack phase is skipped (however, thats no real issue for drum sounds...)
Due to the better results, this option is enabled by default!
Hot tip:: there is a simple trick which allows you to play a bassline and drum sequence in synch during a live session: just add an audio delay unit with t_delay=30 mS (e.g. VST based) to the SID outputs which are playing the basslines in order to compensate the timings.
Thanks in advance.