@@ -598,7 +598,7 @@ export class ShadowCss {
       if (rule.selector[0] !== '@') {
         if (rule.isBlock) {
           const selectorParts = selector.split(_selectorSplitRe);
-          const containsDeep = selectorParts.some((part) => _shadowDeepSelectors.test(part));
+          const containsDeep = selectorParts.some((part) => _shadowDeepSelectorRe.test(part));
 
           selector = this._scopeSelector({
             selector,
@@ -1013,6 +1013,8 @@ const _polyfillHostNoCombinatorRe = /-shadowcsshost-no-combinator([^\s,]*)/;
 // Support for `>>>`, `deep`, `::ng-deep` is then also deprecated and will be removed in the future.
 // see https://github.com/angular/angular/pull/17677
 const _shadowDeepSelectors = /(?:>>>)|(?:\/deep\/)|(?:::ng-deep)/g;
+// Non-global copy for `.test()`, which would otherwise carry `lastIndex` over to the next rule.
+const _shadowDeepSelectorRe = new RegExp(_shadowDeepSelectors.source);
 
 // Splits the selector into independent parts by `,` (comma) unless comma is within parenthesis,
 // for example `:is(.one, two)`. Negative lookup after comma allows not splitting inside nested
 
@@ -465,4 +465,11 @@ describe('ShadowCss nesting', () => {
       '.foo[contenta] .bar { .baz { color: red; } }',
     );
   });
+
+  it('should not let a previous ::ng-deep rule affect the next one', () => {
+    // An empty rule body leaves nothing to reset the global regex's `lastIndex`.
+    expect(
+      shim('.foo ::ng-deep .bar {}\n::ng-deep .baz { .qux { color: red; } }', 'contenta'),
+    ).toEqualCss('.foo[contenta] .bar {} .baz { .qux { color: red; } }');
+  });
 });