I've resolved the problem.
I'll try to explain how. Before upgrading to the latest beta, I had my browser set up with "Use Vertical Tabs". However, I think something had gone wrong, because my 4 pinned tabs were laid out in a vertical stack of icons at the top of the tabs panel, rather than as they ought to be in a horizontal row of icons at the top.
On upgrading, Orion would crash immediately after launch, as described above.
I managed to get Orion running on this profile by erasing windows from the browser_state.plist file. When I had 11 or fewer windows, Orion would run.
Then, while it was running, I changed the sidebar setting to "Use Horizontal Tabs". Changing this resolved the issue, and I could have 12 or more tabs, and successfully re-open Orion.
When I set the sidebar settings back to "Use Vertical Tabs", the pinned tabs were restored to the usual layout. I was able to restore my original browser_state.plist file, and Orion is working as expected.
So, what I think happened is that something to do with the pinned tabs and sidebar got messed up before I upgraded. This interacted poorly with the upgraded code. Flipping the tab orientation fixed what got messed up, so things could work as expected.
Looking at changes in the com.kagi.kagimacOS.plist file, the most significant change between when the pinned tabs appeared incorrectly and I was getting crashes, and when the pinned tabs appeared correctly, is in the Pinned Tabs section. It changed from a dict of UUIDs/URLs
to an array of dicts {id: uuid, url: urlstring}
.
I wonder if this change in the format of saved preferences caused the weird pinned tabs layout, and this in turn caused the layout issue with more than 11 windows?
Anyway, my own problem is now resolved. Thanks so much for looking into it so far.