Build 19042.906 and 19043.906 are available in RPC and Beta Channel

It does not uninstall the previous update.. fails to do so

"it fails with error 0x800f081f if you update from 19043.899, but works fine if you skip the update to .899"

Have you try to uninstall older KB5000842 (released 15th of March) and go back to .867 before update to .906?

New CU's are available together with new Feature Experience Pack.

Releasing Windows 10 Build 19042.906 (20H2) to Release Preview Channel | Windows Insider Blog

The installer is broken. It stops at 20% & then it fails to install.

@michaelleepow90,

Yes, it fails with error 0x800f081f if you update from 19043.899, but works fine if you skip the update to .899 and update directly to .906, 

A fix is coming on the next 21H1 cumulative update.

I bypassed the problem by updating directly from 19043.867 to 19043.906.

Confirm exact issue with a Defender update last night. Couple reboots and it finally resolved itself and installed the definition update.

You could also try kick starting it by manually installing def update:

https://www.microsoft.com/en-us/wdsi/defenderupdates

1349 was the one getting the error, but the next one 1358 updated just fine from W.U.  :-)

B.T.W.

The MS Insider ISO 19043.867 still comes with the legacy MS Edge.

No big deal because the new MS Edge Chromium is the first thing they make you download after a clean install.  

I expect they will fix that on the final MCT ISO release.  :-)

Here's another one for you to check, completed at 20%:

Dism /Online /Cleanup-Image /StartComponentCleanup

I think we have seen that before and somebody explained that if there is nothing else to cleanup it does not go over that 20%.

See here 1st pass went through 100%  and second pass just 20%.