I'm no longer able to scroll using the mouse wheel through my list of symbols in the symbology pane. Was this always the case? I genuinely can't remember, but want to believe you were always able to scroll vertically through symbols using the mouse. I'm on Pro 3.6.1.
This appear to be BUG-000181391 - "Unable to scroll through symbol classes with the mouse wheel when unique values is set as the primary symbology in ArcGIS Pro 3.6."
The current workaround is - "Click and drag the scroll bar or use the arrows at the top or bottom of the scroll bar." It's been fixed in the upcoming ArcGIS Pro 3.7 release.