This bug is marked fixed as of UE4.13: Unreal Engine Issues and Bug Tracker (UE-30481)
It looks like [a workaround][1] was implemented in UE4.13, but it got reverted in UE4.14 because it only supported 1px thick lines. That means it still occurs in UE4.27 and UE5.
For example, curves with sharp corners look like this in the timeline editor:
Can the issue be updated to Unresolved, please?