I just tested and get the same results, so it will take either a revision to Grbl firmware or a change in T2Laser to allow your system to work correctly.
I've opened a Grbl issue for it to see if there's a workaround.
Response from the Grbl developer @chamnit: "Laser mode is not compatible without VARIABLE_SPINDLE. You get no benefit running Grbl this way. As for older boards hardwired for spindle enable on D12, I'm going to have to pass on this. VARIABLE_SPINDLE was introduced a couple of years ago and is the standard build. It's going to take a lot more work than I'd want for laser mode to work with VARIABLE_SPINDLE disabled. It was designed specifically to work with it. At some point, I just have to say no. It's not worth."
I guess we're in uncharted territory now.
Your best bet seems to be modifying the existing board or replacing it, you can get Benbox to work with D11 by changing the pin assignment in settings to make it easy to switch between programs but you would still need to change firmware.