I have (temporarily) resolved the issue by:
- Creating an APFS Volume (e.g., sequoia_v15) with 25GB reserved space and no quota limit
- Downloading the Sequoia 15.0.1 installer (earliest version for me that did not have the issue for external monitor [plus Anker 778 dock]) using command:
softwareupdate --fetch-full-installer --full-installer-version 15.0
- Installing the Sequoia OS to the newly created volume (e.g., sequoia_v15)
- Once the above command completes, go to Applications, and click on Install Sequoia, and select the new Volume created
- After the installation, the Startup Disk was automatically set to the new Volume, so I didn't have to manually set the Startup Disk
- Disable the Software Updates until the issue is resolved by Apple, or a workaround is found
Just to summarize, this issue is not specific to Anker dock IMO - since I use the same dock and external monitor (Dell U3415W) with other laptops without any flickering or blanking issues.
Hope this helps.
Apple - please take a look at this since this is preventing from getting security updates.
Thanks.