It appears PluginMenus have entirely stopped working. I am unable to make a plugin show a PluginMenu at all anymore.
The problem of PluginMenus only showing up when the plugin is loaded during Studio launch has been known and present for a year.
Sample plugin code to reproduce the bug
local menu = plugin:CreatePluginMenu("SampleMenu")
local action = plugin:CreatePluginAction("SampleAction", "Am just a dummy", "Really not of any use")
menu:AddAction(action)
wait(10)
print("showing")
menu:ShowAsync()
Running Windows 10 Professional x64, build 19041.450 / Latest public Studio build.