When you close Start Page when it is the only tab in a Tab Group it causes Orion to exit despite there being other Tab Groups present. It doesn't matter what TabGroup it is i.e. 1st, 2nd, nth, if you close the Start Page when it is the only tab in a Tab Group it causes Orion to exit. I have tried replicating this behavior with non-Start Page tabs like a Kagi search, but it seems to only occur when the tab is a Start Page only tab. Therefore it seems like it is a Start Page related issue .
My guess is there is some dependency parent-child issue going on with Start Page page and the browser itself.
When a new window opens Start Page is opened automatically, similarly when a new Tab Group is opened Start Page is opened autoamtically. When a new window with only Start Page open is closed the window is closed therefore it seems as if the same behavior is being applied to Tab Groups as well.

Closing a tab, Start Page or otherwise, in a Tab Group when it is the only tab, should not close the browser when there are other Tab Groups open.
Version 1.0.4 (143), WebKit 624.1.2.19.2, Tahoe 26.2
Tahoe (26)