Hi @Bastien_Siebman and thank you for flagging this!
Your guess is correct. The reason why you are not receiving a notification when Task A has been completed is because notifications will only be sent for actions or changes performed by others. You don’t receive a notification for an action you have performed yourself.
With that said, I do understand your confusion so I’m going to file a task for our Guide Team to add a Tip on this article to avoid confusion.
I hope it makes sense Bastien but please let me know if you have any follow-up question or comment. Have a nice day!