I apologize for the response delay.
This bug was a side a effect of subtle changes that I posted earlier this year, very difficult to track... I finally could find the time to investigate and fix it: https://github.com/brainstorm-tools/brainstorm3/commit/823b6a5600c0e173a2a3e8d1a9c40072990b09ad
Please update Brainstorm and try again (you'll need to start from the subject creation again).
I am currently updating the text and screen captures in the tutorial page to reflect the last changes:
https://neuroimage.usc.edu/brainstorm/Tutorials/ECoG