Can't untie de Bezier handles do edit it individually. Even click in "untie Handles"

I think the dash-dash line on your key frame makes it untied. You might have to untie the other one. I typically use CTRL-A to select all scoped key frames before clicking the untie icon.

Please submit a bug. If you use "Untie Values" then it works. But note that since selecting the key selects everything, you need to single left click on one of the handle parts again after untying to only move that side of the key.