I'm not sure the bug is in core, and it probably manifests only when you use Views displays provided by other modules, but I remember this happening is various situations, ie. with different contrib displays (and I can't exclude it happened on core displays).
Correct CSS when on display provided by core:
Missing CSS on Views EVA display:
CSS gone missing:
.views-tabs a {
display: inline-block;
padding: 0.625rem;
border: var(--input-border-size) solid #cbcbcb;
border-radius: 0.4375rem;
font-size: small;
line-height: 1.3333;
}
.views-tabs li a {
-webkit-text-decoration: none;
text-decoration: none;
background-color: #fff;
}
.views-tabs li a:hover, .views-tabs li.is-active a, .views-tabs li.is-active a.is-active {
color: #fff;
background-color: #555;
}Closed: duplicate
10.3 ✨
I'm not an expert but this could also be a Claro bug (the core admin theme used here).
This issue was fixed in 🐛 Secondary tabs design is distorted on block view page Active