Confusion with setSerializationInclusion(Include.NON_NULL)

I'm confused with setup in "Usage section":

ObjectMapper mapper = new ObjectMapper();
mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
mapper.registerModule(new JsonNullableModule());

Please take a look on the following snippet ( val=null is passed in JSON and in Map):

record A (JsonNullable<BigDecimal> val) {}
ObjectMapper objectMapper = new ObjectMapper().registerModule(new JsonNullableModule());

A a1 = objectMapper.readValue("{ \"val\" : null }", A.class);
System.out.println(a1);

HashMap hashMap = new HashMap();
hashMap.put("val", null);
A a2 = objectMapper.convertValue(hashMap, A.class);
System.out.println(a2);

Output is consistent

A[val=JsonNullable[null]]
A[val=JsonNullable[null]]

But with setup

ObjectMapper objectMapper = new ObjectMapper().registerModule(new JsonNullableModule())
        .setSerializationInclusion(JsonInclude.Include.NON_NULL);

Output is not consistent

A[val=JsonNullable[null]]
A[val=JsonNullable.undefined]

Well, time for questions:

1. I do not understand why you propose in "Usage" section setup .setSerializationInclusion(JsonInclude.Include.NON_NULL). It looks useless.
2. I do not understand how serialization setup may affect deserialization (objectMapper.convertValue is deserialization)

Start with the Usage section and reproduce both ObjectMapper configurations using the record, JSON input, and HashMap conversion shown in the issue. Trace how the NON_NULL setting affects the documented setup, then update the Usage section so the purpose of the setting and its effect on convertValue are clear, with the observed behavior explained.