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.
Being absolutely unable to solve this issue on my PI-4, I decided to roll-up my sleeves and start all over again.
This time, instead of using Astroberry, I started from scratch using the Ubuntu 21.10 desktop.
I installed all the necessary packages (Indi, Ekos, VNC, etc).
Also, I opted for an external drive (in this case an SSD) instead of an SDCARD (Ubunru 20.04 does not allow that but 21.10 does).
This way I have better perfomances and a lot more space on the drive (at least with the material I had in hand).
After installing everything and swearing from time to time I managed to make almost everything work again.
And guess what: my problem dissapeared. Moreover, my colors are now 14 bits as they should have been, instead of only 8 bits as they were in my previous setup.
I cannot tell which of the modifications hit the nail but one of them did.
However, I am now stuck with a problem I didn't have before and that I will talk about in another post.