Switching between Tab Groups using the Tab Groups drop-down menu leads to the browser being unresponsive and eventually crashing. Each Tab Group only had only one tab in it, which was the Start Page.
I tried to replicate this on a fresh profile, it switches between Tab Groups like normal without crashing.
Version 0.99.138-beta (WebKit 623.1.8.0.0)
macOS Tahoe 26.1 build 25B78