Not a resolution, but a workaround and hopefully a note for Instructure if they need to look into this...I would also recommend reaching out to your Canvas admin team and have them submit a ticket with Instructure directly (have them also reference this post so it can be tracked should other issues come up) as they would escalate the issue with their engineering team the more they get tickets like this.
Workaround
Instead of having the stimulus be on the left and the questions on the right, switch it to stimulus on top with questions below. We use the L/R option like you, so I don't want to change it for the program that uses it (thankfully don't need to worry about it today).
Note for Instructure or others with this issue
This narrow stimulus issue seems to only happen when the length and number of questions exceeds the height of the stimulus. Attached two screenshots for reference. This does NOT appear on the Build menu. Looks like the scroll window isn't generating for the "questions to the right" orientation.