Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/main/resources/csharp/libraries/generichost/JsonConverter.mustache, line 403:

<comment>`oneOf` alternatives that are arrays/maps now generate a `<baseType>JsonConverter` lookup and `WriteProperties` call, so their clients fail to compile or serialize; limit this path to model references and serialize collection alternatives directly before starting an object.</comment>

<file context>
@@ -376,7 +376,39 @@
+            {{^isPrimitiveType}}
+            if ({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}} != null)
+            {
+                {{baseType}}JsonConverter {{#lambda.camelcase_sanitize_param}}{{baseType}}JsonConverter{{/lambda.camelcase_sanitize_param}} = ({{baseType}}JsonConverter) jsonSerializerOptions.Converters.First(c => c.CanConvert({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}.GetType()));
+                {{#lambda.camelcase_sanitize_param}}{{baseType}}JsonConverter{{/lambda.camelcase_sanitize_param}}.WriteProperties(writer, {{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}, jsonSerializerOptions);
+            }
</file context>

Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/main/resources/csharp/libraries/generichost/JsonConverter.mustache, line 404:

<comment>The referenced oneOf model's properties are written directly into the parent's already-open JSON object (flattened), followed by the parent's own WriteProperties. If a oneOf child schema shares a JSON property name with a sibling property on the parent (or with the discriminator/additionalProperties), the serialized output will contain duplicate keys for that name. The included Fruit sample avoids this only by luck (child keys kind/count/sweet vs parent key color); a child such as `apple` defining a `color` property alongside Fruit's `color` sibling would produce duplicate "color" entries, and on deserialization the parent's value silently wins because it is written last. Consider guarding against name collisions (e.g. wrapper/discriminator object or a name-overlap check) or at least documenting this flattening limitation.</comment>

<file context>
@@ -376,7 +376,39 @@
+            if ({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}} != null)
+            {
+                {{baseType}}JsonConverter {{#lambda.camelcase_sanitize_param}}{{baseType}}JsonConverter{{/lambda.camelcase_sanitize_param}} = ({{baseType}}JsonConverter) jsonSerializerOptions.Converters.First(c => c.CanConvert({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}.GetType()));
+                {{#lambda.camelcase_sanitize_param}}{{baseType}}JsonConverter{{/lambda.camelcase_sanitize_param}}.WriteProperties(writer, {{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}, jsonSerializerOptions);
+            }
+
</file context>

Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/main/resources/csharp/libraries/generichost/JsonConverter.mustache, line 386:

<comment>For primitive oneOf members the new code serializes the value as a bare JSON primitive and `return`s before `writer.WriteStartObject()` is reached. This path is asymmetric with the referenced-object path it sits next to: the object path preserves the parent's sibling properties via WriteProperties, but the primitive path writes only the bare scalar and drops every sibling property (and never emits an object). For a composed model that mixes a primitive oneOf member with sibling properties (e.g. `oneOf: [string, Apple]` plus a `color` sibling), serializing when the primitive is selected silently omits the sibling data, so the output won't round-trip. Consider writing the primitive into the object (or documenting that a primitive oneOf cannot be combined with siblings).</comment>

<file context>
@@ -376,7 +376,39 @@
+            {{#isPrimitiveType}}
+            if ({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}} != null)
+            {
+                JsonSerializer.Serialize(writer, {{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}, jsonSerializerOptions);
+                return;
+            }
</file context>