[BUG] Nullable Types Changed [C-Sharp]

I'm not sure if this a bug per-se but I've noticed that since v5.4.0 (at least), date types have stopped being rendered as DateTime? (ie, nullable) and are now rendered simply as DateTime.

I can't find a reference to this change - can anyone illuminate me?

This change resulted in several (minor) code breaks for us.

Here a fragment from the OpenAPI definition, which hasn't changed:

"CustomFieldAdd": {
  "description": "While records provide many fields to track information, organizations often require additional details.",
  "required": [
    "category"
  ],
  "type": "object",
  "properties": {
    "category": {
      "description": "The custom field category. Available values are the entries in the <b>Custom Field Categories</b> tablet.",
      "type": "string"
    },
    "comment": {
      "description": "The comment on the custom field.",
      "maxLength": 50,
      "minLength": 0,
      "type": "string"
    },
    "date": {
      "format": "date-time",
      "description": "The date on the custom field. Uses <a href=\"https://tools.ietf.org/html/rfc3339\">ISO-8601 format:</a><i>1969-11-21T10:29:43</i>.",
      "type": "string",
      "x-supports-datetime-offset": false
    },
    "parent_id": {
      "description": "The parent object's immutable system record ID.",
      "type": "string"
    },
    "value": {
      "description": "The value of the custom field.",
      "type": "object"
    }
  },

Here's the c-sharp code (signature only) rendered by v4.0.3 (which we've used until trying to upgrade to a later version):

public CustomFieldAdd(string category = default(string), string comment = default(string), **DateTime?** date = default(DateTime?), string parentId = default(string), Object value = default(Object))

And here's the code rendered by v5.4.0 and later:

public CustomFieldAdd(string category = default(string), string comment = default(string), **DateTime** date = default(DateTime), string parentId = default(string), Object value = default(Object))

(As an aside, default(type) can be shortened to simply default; the type is inferred. Also, date is preferred over DateTime.)

UPDATE

After some more digging, I see that other types are affected as well, such as int and bool. Eg:

v4.0.3:

public ApiList(string id = default(string), string name = default(string), string description = default(string), **int?** recordCount = default(**int?**), DateTime? dateModified = default(DateTime?), string lastModifiedByUserName = default(string), string lastModifiedByUserId = default(string), **bool?** isPublic = default(**bool?**))

v5.4.0:

public ApiList(string id = default(string), string name = default(string), string description = default(string), **int** recordCount = default(**int**), DateTime dateModified = default(DateTime), string lastModifiedByUserName = default(string), string lastModifiedByUserId = default(string), **bool** isPublic = default(**bool**))

Start by comparing C# output from generator v4.0.3 and v5.4.0 using the OpenAPI fragment in the report, focusing on nullable date, integer, and boolean parameters. Done means identifying the change that removed nullable types and either restoring the prior output or documenting why the generated signatures should differ.