Hmmm... I dunno. I have an RD-700NX, and an old RD-300s. The RD-300s sends CC123, All Notes Off, at last key up which causes trouble with some VSTis that don't handle it properly in connection with sustain (e.g. NI Akoustic and Alicia's Keys), but TTS-1 is not affected. And newer Roland keyboards don't send it any more. Nevertheless, you should try running a MIDI port monitor like MIDI-OX on your input port, and make sure the keyboard isn't sending anything odd.
I also used to run an 1820M, and its MIDI ports were the fastest (PCI-based) I've ever used so that shouldn't be a problem.
MIDI buffering shouldn't be an issue, either, as it's not active when you're playing live. It only affects buffering audio from soft synths driven by recorded MIDI.
Sure sounds like something going on specifically with TTS-1, but I can't imagine what that would be. This affects all patches?
P.S. I've tried all kind of combinations of both RDs sending simultaneously on multiple channels with the TTS-1 receiving All Ports Omni, and I can't produce anything unexpected. The last held note always continues to sustain unless one keyboard sends a Note Off while the other one is holding the same note number, which makes perfect sense but should not be possible with input from only one controller.