Hi,
my Avast Free Antivirus (German ed., os: W7 Pro) had been updated at least twice since the Feb. 12 update (build 21.1.5968.561), most recently on Feb. 18 (build 21.1.5968.630).
I assume that these were the "micro-updates" which rocksteady mentioned (#44)?
for me, micro-updates were followed by the "what's new with 21.1" screen after rebooting my computer.
It seemed to me that avast had solved this problem few versions ago and that the screen should only appear after a main update and not after a simple micro update like now ?
I may be mistaken to have read this few monthes ago 
Micro updates (possibly a bug fix) can be delivered as a part of the Virus definitions and "engines" these generally wouldn't get any program update notification, as it isn't specifically a program update in the true sense.
I wanted to clarify this before opening Avast again but it's impossible
- to change the update setting from "Ask when ... available" to "Manual update" and
- to deactivate Avast in the autostart.
@ vitesa
If it is your intention to disable avast at startup (my bold text in the quote), you are going to have bigger issues than small micro updates. Avast is designed as a resident on access scanner and crippling it leaves you at risk. Doing that would mean you would get zero update notifications as it wouldn't be running.
You could however switch Avast to Passive mode, but as mentioned you would be at risk and you wouldn't be getting any updates micro, virus definitions or otherwise. You would in essence require another resident scanner installed or you would have no protection.
Personally I don't pre-empt problems, rather I only react-respond to problems, life is too short to turn over every leaf to see if there is something untoward under it.
I have only ever seen these "Whats New popups" directly after a Program Update, after which a system restart is required to apply the changes. I have not had this after any micro-update. However on my win10 laptop I rarely restart it but just put in into hibernation (close the lid).