Hello, not sure if this is the "normal" behavior or not : when a registration status change from Waitlist to Complete (via autofill option), no mail is sent.
I would have expect that the (Registration) Confirmation email would be sent at this moment, as only the Wait list confirmation email was received previously.
Create an event/node with a registration capacity with 1. Enable waitlist, check Autofill available spots.
Then register with account A (confirmation email is well received). Then register with account B (waitlist email is well received). Then cancel the registration of A. Thus the registration of B change from Waitlist to Complete, but no confirmation email is received.
Send the confirmation email when registration status change from Waitlist to Complete.
Active
3.4
Registration Wait List
Please double check this. Perhaps the second email was trapped in a spam filter. I ran your Steps to reproduce in a test system, and it worked perfectly, with the second confirmation email sent as soon as the auto fill completed. The emails are sent to the registrant.