Moodle: 3.9.2+ (Build: 20201023)
Grid Course format: 220.127.116.11
Lambda theme: 1.98.14
The attached screenshot has "Set the section title summary background opacity on hover" set to 1 and I hovered over two titles. Neither title summary disappears when I am no longer hovered over the title.
The title summaries work as expected using both the Boost and Classic themes for all settings of "Set the section title summary background opacity on hover"
The behavior has been repeated on multiple windows systems and in most recent versions of both chrome and firefox.