I have a mask w/ a dozen vertices that I've keyframed across my timeline. After adjusting the vertices I realize I only need the top half of them to do the job. I want to delete the bottom half that are unnecessary, which I have never needed to do before. I knew there was a preference that was related to this, so I checked the option "Preserve Constant Vertex and Feather Count when Editing Masks" and verified that it was UN-checked, since I am intending to change the vertex count. To my surprise when I selected the points I wish to remove, they were only removed until the next keyframe of the mask path. I tried a few different spots along the timeline but observed the same behaviour. I checked the AE reference manual (https://helpx.adobe.com/after-effects/using//animating-shape-paths-masks.html#designate_the_first_ve...) and it included a note that confirmed what I expected to happen.
There are no specific mentions of keyframes so I take the phrase "throughout the duration of the path." to include across keyframes. I checked YouTube but found nothing relevant as a stand-alone topic. I also checked this support forum to see if anybody had discovered a bug that was impacting this functionality. So, is this user error or a bug? Thanks in advance!