I tried the Canon DSLR driver and I have exactly the same problem.
Also, the BULB option is ON, and my Canon is also set to BULB. Beyond 2 seconds everything works fine with the exposure. Bellow 2 seconds I use M instead of B and it works all right too (with Bulb option OFF in INDI).
What is really weird is the problem does not appear when controling my PI in remote.