Ugh, of course it does not perform body validation in ValidateJSONBody(jsonBody []byte) error if the schema is not found in rest.ParamInBody or the schema is nil:

func (v *Validator) ValidateJSONBody(jsonBody []byte) error {
  name := "body"
  
  schema, found := v.inNamedSchemas[rest.ParamInBody][name]
  if !found || schema == nil {
    return nil
  }
  // ...
}

It only seems to have this value if a constraint has been given to a type. In my situation it was using uint for a field that automatically set a >=0 constraint and therefore a schema was found. Changing that to int and just using a JSON tag does not make this implementation possible.

For example:

// No validation will happen.
type ExampleIn struct {
  Value int `json:"value"`
  Name string `json:"unit"`
}

// This does get validated.
type ExampleIn struct {
  Value int `json:"value" minimum:"1"`
  Name string `json:"unit"`
}

Maye we could add a way to have schema data even if it is trivial?
For example in collector.go in func (c *Collector) jsonSchemaCallback(validator rest.JSONSchemaValidator, r openapi.Reflector) openapi.JSONSchemaCallback {}:

// ...
// It'd be great to have schema data even if it is trivial.
if schema == nil || schema.IsTrivial(r.ResolveJSONSchemaRef) {
  if err := validator.AddSchema(rest.ParamIn(in), paramName, nil, required); err != nil {
    return fmt.Errorf("add validation schema %s: %w", loc, err)
}
  
  return nil
}

Then, either always validate if a schema is not nil - which does indeed affect performance if it has to go through every check although it is not necessary, or implement a separate validation for just decoding JSON bytes into a structure.