diff --git a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
index 9e7b59a0cfac..f6ad61148208 100644
--- a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
+++ b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
@@ -401,7 +401,8 @@ class ClassfileParser(
       classRoot.setFlag(sflags)
       moduleRoot.setFlag(Flags.JavaDefined | Flags.ModuleClassCreationFlags)
 
-      val privateWithin = getPrivateWithin(jflags)
+      val jflags1 = innerClasses.get(currentClassName.toString).fold(jflags: Int)(_.jflags)
+      val privateWithin = getPrivateWithin(jflags1)
 
       classRoot.setPrivateWithin(privateWithin)
       moduleRoot.setPrivateWithin(privateWithin)
diff --git a/compiler/src/dotty/tools/dotc/printing/Formatting.scala b/compiler/src/dotty/tools/dotc/printing/Formatting.scala
index a36e6f48533a..14b822b11333 100644
--- a/compiler/src/dotty/tools/dotc/printing/Formatting.scala
+++ b/compiler/src/dotty/tools/dotc/printing/Formatting.scala
@@ -109,6 +109,13 @@ object Formatting {
           case Atoms.Range(lo, hi) => CtxShow(s"Range(${toStr(lo.toList)}, ${toStr(hi.toList)})")
       end given
 
+      given Show[ast.untpd.Modifiers] with
+        def show(x: ast.untpd.Modifiers) =
+          CtxShow(s"Modifiers(${toStr(x.flags)}, ${toStr(x.privateWithin)}, ${toStr(x.annotations)}, ${toStr(x.mods)})")
+
+      given Show[ast.untpd.Mod] with
+        def show(x: ast.untpd.Mod) = CtxShow(s"Mod(${toStr(x.flags)})")
+
       given Show[Showable]                            = ShowAny
       given Show[Shown]                               = ShowAny
       given Show[Int]                                 = ShowAny
diff --git a/tests/pos/i21631_joint/AbstractChannel.java b/tests/pos/i21631_joint/AbstractChannel.java
new file mode 100644
index 000000000000..fbcd04549def
--- /dev/null
+++ b/tests/pos/i21631_joint/AbstractChannel.java
@@ -0,0 +1,7 @@
+public abstract class AbstractChannel {
+    protected AbstractChannel() {}
+    protected abstract AbstractUnsafe newUnsafe();
+    protected abstract class AbstractUnsafe {
+        public abstract void connect();
+    }
+}
diff --git a/tests/pos/i21631_joint/i21631.scala b/tests/pos/i21631_joint/i21631.scala
new file mode 100644
index 000000000000..c567d75b1375
--- /dev/null
+++ b/tests/pos/i21631_joint/i21631.scala
@@ -0,0 +1,5 @@
+class Channel extends AbstractChannel() {
+  override def newUnsafe(): AbstractChannel#AbstractUnsafe = new AbstractUnsafe {
+    override def connect(): Unit = ???
+  }
+}
diff --git a/tests/pos/i21631_separ/AbstractChannel_1.java b/tests/pos/i21631_separ/AbstractChannel_1.java
new file mode 100644
index 000000000000..e89767d6d73e
--- /dev/null
+++ b/tests/pos/i21631_separ/AbstractChannel_1.java
@@ -0,0 +1,7 @@
+public abstract class AbstractChannel_1 {
+    protected AbstractChannel_1() {}
+    protected abstract AbstractUnsafe newUnsafe();
+    protected abstract class AbstractUnsafe {
+        public abstract void connect();
+    }
+}
diff --git a/tests/pos/i21631_separ/i21631_2.scala b/tests/pos/i21631_separ/i21631_2.scala
new file mode 100644
index 000000000000..89524e333920
--- /dev/null
+++ b/tests/pos/i21631_separ/i21631_2.scala
@@ -0,0 +1,5 @@
+class Channel extends AbstractChannel_1() {
+  override def newUnsafe(): AbstractChannel_1#AbstractUnsafe = new AbstractUnsafe {
+    override def connect(): Unit = ???
+  }
+}