Thank you for reporting a bug to Unity.
After reviewing the behavior, we've confirmed it aligns with the current design and intended use of the feature. We understand this may differ from your expectations or workflow.
The original implementation of zoomable areas within Unity allow for zooming and panning outside of the range of animatable content. When doing so, the maximum range is extended to the current visual range the window is being scrolled to. However, this maximum range is shrunk back when dragging the slider bar left. This is the intended behaviour as the slider bar is outside of the range of animatable content.
This seems like it is working when there are no keyframes or animation events, but that's because the range of animatable content is artificially set to [0, 1]. Going outside of these bounds will produce the same behaviour.