That tail is flying behind two wings. One high above it, one somewhat below it. Both of which are creating downwash AND airflows that affect each other's airflows. Lord knows what that tail sees for airflows. But that tail is having to deal with the pitching moments of TWO cambered wings. And it's doing it in a very different environment than a monoplane's tail sees.

BTW, the lift needed for our models is really very easily provided by most cambered wings so they seldom assume much AOA to provide it. Increase the load by screwing up the AOI of the tail, and the induced load and drag increase still probably won't be enough that the wing needs to compensate with lots more AOA. So the tail down cruise is probably more the result of AOI being off than anything else.

But heck, this stuff really isn't sound byte easy.

No mention has been made of the engine's angles.