The button indicates the last tab group you opened by default. We added that a few updates ago as people requested it.
As soon as you change folders or open or close one tab, you're no longer strictly looking at the same tab group anymore, so it only makes sense to indicate the one you loaded/started from.
Re freezes, maybe due to particular files or folders triggering a problem (potentially in a 3rd party shell extension):
If the process freezes, you can generate a dump using Task Manager: