Custom fields not showing up in self-hosted version

[EricKean]

Hi,

I think (?) I'm following the documentation in creating custom fields for an LTI 1.3 app:

{
"title": "Adapt",
"scopes": [
"https://purl.imsglobal.org/spec/lti-ags/scope/lineitem",
"https://purl.imsglobal.org/spec/lti-ags/scope/lineitem.readonly",
"https://purl.imsglobal.org/spec/lti-ags/scope/result.readonly",
"https://purl.imsglobal.org/spec/lti-ags/scope/score",
"https://purl.imsglobal.org/spec/lti-nrps/scope/contextmembership.readonly"
],
"extensions": [
{
"platform": "canvas.instructure.com",
"settings": {
"platform": "canvas.instructure.com",
"placements": [
{
"placement": "link_selection",
"message_type": "LtiDeepLinkingRequest",
"target_link_uri": "https://dev.adapt.libretexts.org/api/lti/configure"
},
{
"placement": "assignment_selection",
"message_type": "LtiDeepLinkingRequest",
"target_link_uri": "https://dev.adapt.libretexts.org/api/lti/configure"
}
],
"link_selection": {
"placement": "link_selection",
"message_type": "LtiDeepLinkingRequest",
"target_link_uri": "https://dev.adapt.libretexts.org/api/lti/configure"
},
"assignment_selection": {
"placement": "assignment_selection",
"message_type": "LtiDeepLinkingRequest",
"target_link_uri": "https://dev.adapt.libretexts.org/api/lti/configure"
}
}
}
],
"public_jwk": {
~an actual key~
},
"description": "Adapt-2",
"custom_fields": {
"custom_canvas_api_domain": "$Canvas.api.domain",
"custom_canvas_course_id": "$Canvas.course.id",
"custom_canvas_user_id": "$Canvas.user.id",
"user_image": "$User.image",
"membership_roles": "$Canvas.membership.roles",
"custom_canvas_account_id": "$Canvas.account.id",
"custom_canvas_user_is_root_account_admin": "$Canvas.user.isRootAccountAdmin",
"custom_canvas_root_account_id": "$Canvas.rootAccount.id",
"custom_canvas_root_account_uuid": "$vnd.Canvas.root_account.uuid",
"account_name": "$Canvas.account.name",
"custom_external_tool_url": "$Canvas.externalTool.url"
},
"target_link_uri": "https://dev.adapt.libretexts.org/api/lti/redirect-uri",
"oidc_initiation_url": "https://dev.adapt.libretexts.org/api/lti/oidc-initiation-url"
} 

However, upon launching, https://purl.imsglobal.org/spec/lti/claim/custom returns an empty array:

-jwt: array:2 [▼
    "header" => array:3 [▶]
    "body" => array:27 [▼
      "https://purl.imsglobal.org/spec/lti/claim/message_type" => "LtiResourceLinkRequest"
      "https://purl.imsglobal.org/spec/lti/claim/version" => "1.3.0"
      "https://purl.imsglobal.org/spec/lti/claim/resource_link" => array:5 [▶]
      "https://purl.imsglobal.org/spec/lti-ags/claim/endpoint" => array:5 [▶]
      "aud" => "10000000000028"
      "azp" => "10000000000028"
      "https://purl.imsglobal.org/spec/lti/claim/deployment_id" => "32:20d4c62c24a96e1f3afb75776a253004109a1e22"
      "exp" => 1629122455
      "iat" => 1629118855
      "iss" => "https://dev-canvas.libretexts.org"
      "nonce" => "nonce-611a6188f00478.76082033"
      "sub" => "382a91b9-a622-45fa-96be-966647be198f"
      "https://purl.imsglobal.org/spec/lti/claim/target_link_uri" => "https://dev.adapt.libretexts.org/api/lti/redirect-uri"
      "picture" => "https://dev-canvas.libretexts.org/images/messages/avatar-50.png"
      "email" => "me@me.org"
      "name" => "me@me.org"
      "given_name" => "Me"
      "family_name" => ""
      "https://purl.imsglobal.org/spec/lti/claim/lis" => array:4 [▶]
      "https://purl.imsglobal.org/spec/lti/claim/context" => array:6 [▶]
      "https://purl.imsglobal.org/spec/lti/claim/tool_platform" => array:6 [▶]
      "https://purl.imsglobal.org/spec/lti/claim/launch_presentation" => array:7 [▶]
      "locale" => "en"
      "https://purl.imsglobal.org/spec/lti/claim/roles" => array:6 [▶]
      "https://purl.imsglobal.org/spec/lti/claim/custom" => []
      "errors" => array:1 [▶]
      "https://purl.imsglobal.org/spec/lti-nrps/claim/namesroleservice" => array:4 [▶]

Any help would be greatly appreciated.