[Baertram]

Just a guess:

Are you testing with ONLY your addon active? If not: Do so please!
Other addons could intercept the messages of the chat.
e.g. LibDebugLogger -> Will show the d messages in the DebugLogViewer addon (if you got that one installed).

After reviewing your code:
EVENT_ADD_ON_LOADED is fired BEFORE the chat is ready!
So you need to use EVENT_PLAYER_ACTIVATED to see the chat messages.
Or an addon like pChat which enables to read these messages in the system chat channel as well.

OR you use the library mentioned above.
LibDebugLogger + the addon DebugLogViewer.

Once enabled go to the settings of DebugLogViewer and enable the Stack traceback setting and output of the messages you send via d() before the chat is ready, as well as a list of the loaded addons and their order, should be shown in the quick log and the debug log viewer UI.