You're right.
Do you have any idea why 86% of francophones give evidence in English? Is it because the chairs don't provide any guidelines on this?
Here at the Standing Committee on Official Languages, most of the discussions are in French. It's difficult to imagine what it's like elsewhere, but it would appear to be the other way around.
People seem to switch into English as soon as there's a technical glitch. As most of the committee chairs do not speak French, this has much more of an impact on francophones.
Could the solution be to have conditions that would encourage witnesses who want to give evidence in French to do so?