I'm noticing this as well. I have a few saved windows containing pinned tabs. Without warning, many long-closed tabs will suddenly reappear to the right of the pinned tabs, as if they were never closed, for two of these saved windows. The issue can be reliably reproduced by quitting and restarting the browser (or when it crashes). On at least one occasion the tabs all reappeared during the session without having restarted the browser (I had locked the screen and stepped away from the laptop for 30 minutes).
EDIT to add: earlier today I deleted several saved windows. The browser crashed, and when I restarted it, the saved windows magically reappeared in the sidebar (with all the previously-closed tabs).
Version 0.99.131-beta (WebKit 621.1.2.111.4)
Build date Feb 13 2025
MacBook Air (macOS Sequoia 15.3 build 24D60)