Storyline Screen Recording is capturing the whole screen instead of the defined area

[Note: I'm running StoryLine 3 on a Windows 10 laptop]

I am working on updating some systems training and went to record my screen in Storyline using the Slides > Record Screen > Record Your Screen... option. It opened the screen recording window. I adjusted it to fit the FireFox window I wanted to record and clicked the Record button. When the 3-2-1 countdown starts, I noticed that it was showing the bottom left corner of my screen, not the screen capture area I'd defined. I recorded a few seconds, and stopped. The preview in StoryLine where I'm asked how I want to place my recording shows that the recording did grab my entire screen, plus a significant black bar below it. (see attached)

I have uninstalled, restarted and re-installed Storyline, but it's still doing the same thing. I have tried different options for screen resolution (4:3, 9:16, window) and get the same result each time. My GoogleFu is failing me. Can anyone here help?