BUG: Pickwhip in expression editor
When Pickwipping a slider inside and expression with brackets, the auto expression will miss the closed bracket because of the outside brackets. This means you need to add another bracket on the end which is fine, but some people may not see it get missed until an error spits out.

Hi Josiah,
Thank you very much for reporting this issue. As of After Effects’ March 2021 release ( version 18.0 ), pick-whipping between brackets properly preserves all existing brackets and no longer results in an expression that needs correcting. Please let us know if you continue to see any issues with pick-whipping in the expression editor after updating to AE 18.0.
https://helpx.adobe.com/after-effects/user-guide.html/after-effects/kb/fixed-issues.ug.html
- John, After Effects Engineering Team