The combination of padding, negative-margin, and sticky makes hit testing be off. In the test case below, hover over the sticky element and nothing happens. Over 50px below the sticky element and the hover highlighting takes place. Similarly, click events don't fire, so it's clearly the hit testing and not something specific to hover.
 
<style>
.container {
  padding: 50px;
}
.sticky {
    margin-top: -50px;
    position: sticky;
    top: 0px;
}
.sticky:hover {
  position:sticky;
background-color: red;
}
</style>

<div class="container" style="border: 5px solid pink; height: 300px; overflow: auto;">
<div class="sticky">sticky</div>
<div style="height: 1000px"></div>
</div>

http://plexode.com/eval3/#s=aekVQXANJVQMbAx14Hz1PD0RQT1VCSk9GUwFcmAEBhRsBEhdRWRyYXpgPVFVKRExapKanAYYOfqoOrK6wT70Bfaq1t7nGyMEBEcWxs8y4WhtJUFeiu8fJG9fOmIsOiKpTRkXGsk8dEJaYmB1yAXcePQObnZ+hKb6AJ4fIDQiqCyuAKHAJRUBMDgBQaqCgRaavAJ8VRCKxVKCwAaXd72fD6cZafQfkzwOUhesCgkSVcVi0olctCEsAZeAA