No 'label' or 'icon' property when defined in custom class

My custom class: https://github.com/ExpidusOS/shell/blob/master/share/expidus-shell/expidus/widgets/app-button.lua

local lgi = require('lgi')
local Gtk = lgi.Gtk
local GObject = lgi.GObject
local Expidus = lgi.package('Expidus')

Expidus:class('AppButton', Gtk.FlowBoxChild)

Expidus.AppButton._property.label = GObject.ParamSpecString('label', 'Application label', 'Label', 'App', { 'READABLE', 'WRITABLE', 'CONSTRUCT' })
Expidus.AppButton._property.icon = GObject.ParamSpecString('icon', 'Application icon', 'Icon', 'app-icon-unknown', { 'READABLE', 'WRITABLE', 'CONSTRUCT' })

function Expidus.AppButton:_init()
end

function Expidus.AppButton:_class_init()
end

return Expidus.AppButton

Code that calls my class: https://github.com/ExpidusOS/shell/blob/master/share/expidus-shell/expidus/launcher.lua

ExpidusAppButton{
  label = entry.label,
  icon = entry.icon
}

Result:

2020-11-03 19:57:07 E: awesome: Error during a protected call: ./expidus/launcher.lua:55: Expidus.AppButton: no `label'
stack traceback:
        [C]: in function 'error'
        /usr/share/lua/5.3/lgi/component.lua:160: in function </usr/share/lua/5.3/lgi/component.lua:156>
        [C]: in metamethod '__newindex'
        ./expidus/launcher.lua:55: in upvalue 'callback'
        /usr/share/awesome/lib/menubar/menu_gen.lua:128: in function </usr/share/awesome/lib/menubar/menu_gen.lua:93>
        (...tail calls...)
        /usr/share/awesome/lib/menubar/utils.lua:409: in function </usr/share/awesome/lib/menubar/utils.lua:406>
        [C]: in function 'xpcall'
        /usr/share/awesome/lib/gears/protected_call.lua:36: in function </usr/share/awesome/lib/gears/protected_call.lua:35>
        (...tail calls...)

Note that I've reduced the calling code down so I don't have the entire file pasted.

Start with share/expidus-shell/expidus/widgets/app-button.lua and share/expidus-shell/expidus/launcher.lua to reproduce the custom-class initialization, then inspect /usr/share/lua/5.3/lgi/component.lua around the reported property assignment. Done means custom label and icon properties are accepted when ExpidusAppButton is constructed, without the reported no-property error.