Did you find the cause of this strange behavior? I tried to reproduce the problem on Moodle versions 3.7.1 and 3.8 (Moodle QA Testing Site), but without success.
Did you clear the site caches (Site Administration / Development / Purge caches)?
Do you use a custom theme? If so, does it correctly define the 'secure' layout type? Can you try your quizzes on a site with a standard theme, Boost or Classic, and see if the problem persists?
Thank you for the reply. I believe that the problem occurs when using the "classic" theme-- after the latest Moodle update, I noticed that the question layout was a little bit strange with this particular theme. I ended up changing the theme to "boost" and have not had the problem since.
I use Boost, just like Moodle QA Testing Site. I had no problem with this theme that I consider excellent.
I'm glad you solved the problem!