I would also consider dropping or extending the arbitrary cutoff date for past edits; users who have not edited in the last few months are going to be no less confused by the tabs disappearing (and in fact much less likely to see any messaging that happens outside the VE interface).

The system defaults to your "last editor". It doesn't not mean you had saved an edit in that editor. If you just launched VE to see or test something, and then left the page, that counts too. I have campaigned for launched or opened, to make that extra clear. I believe it may default to VE if you haven't edited in a while though (and that may depend on a cookie limit?).

I tried it on testwiki where I happened to have some recent wikitext edits, and got the same result. While it is possible that I opened and left VE on all three wikis since my last edit, it is very unlikely. So I think that feature doesn't work quite right. (And I still think defaulting to split tabs would be less confusing - no mysterious UI change.)

Also I wonder if it's a good idea to set the default when someone opens VE and then leaves. Chances are that means they didn't enjoy it.

If you were logged-in on test2.wiki, sure you didn't reset your preferences lately?

I never do that. Also I encountered this today on officewiki, where I also only used wikitext mode recently (even in Flow). Pretty sure this doesn't work the way you describe it.