JSFiddle - React, Tailwind, and code Playground
HTML
<!-- Как есть -->
<div>
<span>Span 1.</span>
<strong>
<span>Span 2.</span>
</strong>
</div>
<span>Span 2.</span>
<!-- Как должно быть -->
<div>
<span>Span 1.</span>
<strong>
<span>Span 2.</span>
</strong>
<span>Span 4. Соседниний спан</span>
</div>
<span>Span 2.</span>
CSS
strong + span {
text-decoration: underline;
}