paper-mc/paper-server/patches/sources/net/minecraft/util/datafix/DataFixers.java.patch

--- a/net/minecraft/util/datafix/DataFixers.java
+++ b/net/minecraft/util/datafix/DataFixers.java
@@ -515,6 +_,24 @@
         Schema schema44 = builder.addSchema(1456, SAME_NAMESPACED);
         builder.addFixer(new EntityItemFrameDirectionFix(schema44, false));
         Schema schema45 = builder.addSchema(1458, V1458::new);
+        // CraftBukkit start
+        // API allows setting player custom names, so we need to convert them.
+        // This does *not* handle upgrades in any other version, but generally those shouldn't need conversion.
+        builder.addFixer(new DataFix(schema45, false) {
+            @Override
+            protected TypeRewriteRule makeRule() {
+                return this.fixTypeEverywhereTyped("Player CustomName", this.getInputSchema().getType(References.PLAYER), typed -> {
+                    return typed.update(DSL.remainderFinder(), dynamic -> {
+                        final String customName = dynamic.get("CustomName").asString("");
+                        if (customName.isEmpty()) {
+                            return dynamic.remove("CustomName");
+                        }
+                        return dynamic.set("CustomName", LegacyComponentDataFixUtils.createPlainTextComponent(dynamic.getOps(), customName));
+                    });
+                });
+            }
+        });
+        // CraftBukkit end
         builder.addFixer(new EntityCustomNameToComponentFix(schema45));
         builder.addFixer(new ItemCustomNameToComponentFix(schema45));
         builder.addFixer(new BlockEntityCustomNameToComponentFix(schema45));