In the recent past, we have seen the Windows 11 Taskbar receive a ton of updates in an attempt to enhance the user experience. The most recent one is a new overflow menu designed to make it easier for users to launch and switch apps when space is limited.
A Reddit user u/caipira113 spotted what appeared to be a new undocumented feature, rounded corners in the Taskbar (via Neowin). The rounded Taskbar appeared for the user upon downloading Windows 11 build 25174.1000 from the Dev Channel. They had earlier alluded that this was the same case with the 25131 build which featured several Microsoft Store updates but this time around there's evidence to back up this claim as shown below.
However, Brandon LeBlanc, a Senior Program Manager on the Windows Insider Program Team through a tweet has come out and confirmed that this "looks like" a bug and that they are not conducting any A/B testing.
This looks like a bug. There are a few bugs like this that change the UI in various ways. It's not something we're doing any A/B testing on.
— Brandon LeBlanc (@brandonleblanc) August 5, 2022
The Reddit user further indicated that the rounded corners appeared randomly and did not disappear even after trying to update the display driver. It however disappeared after restarting explorer.exe via Command Prompt. As indicated by Brandon, this just seems like a bug that alters the normal functioning of the user interface.