Yep (for testing), but it got released for all users yesterday.
This update was released to Windows Insiders last week to test before being made publicly available today.
As you can see from my screenshot, not everyone.
Just upgraded via Media Tool HP Envy, decided not to wait for Windows update to offer 1903
Took several reboots to get things settled down.
Avast first reboot came up with both Behavior Shield and Ransom Shield NOT Enabled.
A couple of reboots resolved that, however then Core Shields showing needing to be Installed. Another reboot resolved that and all seems good now. :-\
Going to Upgrade HP Compaq now. Expecting the same rebooting to settle things down.
https://forum.avast.com/index.php?topic=156141.msg1506675#msg1506675
You might want to try disabling the Avast Self Defense Module before doing the update to see if that makes a difference.
I did specifically because of your previous upgrade experience. 
Lately I’ve found the majority of issues I’ve had with Avast have resolved with more then one reboot. Sometimes 3 or 4.
Further information
Just finished upgrading to 1903 on a second machine (HP Compaq) and got a BSOD on one of the final stages. The Upgrade ‘installation had to be Recovered’ then continued on; rebooting 3 more times.
All Avast shields were enabled after the upgrade, no Apparent issues with Avast on this machine.
Older Windows 10 Versions Get Intel Microcode Updates for MDS Vulns
https://www.bleepingcomputer.com/news/microsoft/older-windows-10-versions-get-intel-microcode-updates-for-mds-vulns/
It would make more sense to update the older Windows 10 version since the latest version of Windows 10 (1903) is not affected.
Microsoft Prepares to Autoupdate Windows 10 v1803 and Earlier Devices
https://www.bleepingcomputer.com/news/microsoft/microsoft-prepares-to-autoupdate-windows-10-v1803-and-earlier-devices/
I personally think it would be nice to see everyone on the same version. It certainly makes offering help easier.
If only it was that easy, for absolutely ages I have been waiting diligently to get a feature update to my 1803 version. So it is about time MS did something about this.
Just received the 2019-06 Accumulative update KB4501375 on both machines.
Seems odd on a Friday.
The update was also unusually lengthy it seemed.
Maybe it’s just me. :-\
It does seem strange as there seems to be more KBs outside of the usual Patch Tuesday, I just checked and there was a cumulative one awaiting download/installation. Not the same as yours, but I’m still on 1803 awaiting MS getting their backside in gear and do the promised Feature update to 1903.
Yes, I’ve been checking regularly over the past several months and yes have found updates on different days other than Patch Tuesday.
I still don’t have the update mentioned in this post.
Maybe it’s waiting for Tuesday to send it to me ???
I never understood the idea of patch Tuesday as not everyone can get it on Tuesday given the user base for all supported versions of windows. This can only be harder when we are talking about feature updates.
The MS update servers must be absolutely massive.
In case you’re interested, here’s a list of them:
https://social.technet.microsoft.com/Forums/en-US/b3327a8c-88e0-4b7e-a1be-446498f726d8/list-of-update-server-addresses?forum=winserverwsus
Absolutely huge when you consider all of those * asterisks in the list in that article. I’m sure MS also used (or used to use) other companies for update delivery, Akamai cloud distribution services comes to mind.
Not just you. Updates are now appearing whenever MS wants them to.
In addition, this update hung, it downloaded to 100% but never began to install. So, I rebooted and then it started installing. And, like you, it took a while to complete, over an hour, easily double the usual time.
Reboot being required in the middle of doing a security update was unusual, to say the least.
Windows 10 SFC /scannow Can’t Fix Corrupted Files After Update
https://www.bleepingcomputer.com/news/microsoft/windows-10-sfc-scannow-cant-fix-corrupted-files-after-update/