I think title explains it. In Event List Block the “allow updates” option is selected which makes the subject column a link that you can click to bring up the “event details” pane for that event. The tool-tip is shown over that pane.
Can’t do a screen shot as it disappears.
Using Firefox Beta 107.0b4 (64-bit)
Occurs in Google Chrome as well Version 106.0.5249.119 (Official Build) (64-bit)