(I do not have a reproducible recipe for this one -- it was happening to me 100% consistently, then I restarted and the problem went away. But for the record🙂
I run Orion pretty hard: I have probably a hundred+ open tabs, scattered across four full-screen windows on two monitors. Each window has a distinct purpose. One of them is my "everyday" window, where I keep Gmail as a pinned tab. (One is a named "Reading" window, which doesn't seem to be involved, but I note that this problem was specifically happened between two of the unnamed physical windows, which might be relevant.)
Yesterday, I updated to 0.99.133-beta. After doing so, I found that clicking on links in Gmail didn't seem to open a new tab. After worrying about this for a couple of minutes, I remembered that I'd seen similar behavior before, a few months ago. So I went hunting around, and found that those links were opening in my "work" window, instead of in the "everyday" window where I was clicking on the links.
This behavior persisted, and was completely consistent: I would click on a link in an email in the pinned Gmail tab in the "everyday" window, and the new tab would open (always in the same place in the tab order) in the "work" window.
I experimentally restarted Orion completely, and the problem vanished, and hasn't recurred since.
I expect new tabs to always open in the same window as their source links.
Version 0.99.133-beta (WebKit 621.1.2.111.4)
Sequoia (15)