If you select the Layer (in the Layers palette, in much the same way you select Groups, paths, whatever) and then look at the Appearance palette, you may find that you have a Stroke applied there, at layer level.
Checked it out and nothing I'm afraid./ Thanks for the idea (y)
Can you look in the Appearance panel if there is an additional stroke?
I couldn't find anything, I've restarted my laptop and drawn it again. It's drawn it without an outline this time. No idea what was happening still 😞
Can you share an example file? It's difficult to debug without being able to play with the file and experiment.