Reply from Support:
We’ll zip up our GRBL build in the near future and post it. We made minimal changes to it from the latest official release, mainly to shoe horn the dual Y homing into a 328.
You don’t need to tell us about Sonny, we were the ones that convinced him to charge companies a “support fee” based on machine sales. We paid him a small fortune to fund GRBL development (and contributed code) until he moved on from it.
Regarding the parenthetical comments, that’s not a thing in GRBL, it’s in the USB chip we use. We don’t test our machines with other senders, so it’s not a code path that got tested. The next time we’re in that code, we’ll take a look at it.
Looks like the bug isn’t in an open source component, so we’ll need to wait for Carbide 3d to fix it.