ORIGINAL: Ormand
Actually, the new model pointing to -1 is not a problem. When you start with a model, say it is the Light Tank Faust, the model doesn't actually have a path to upgrade to yet, so it to will point to -1 for the upgrade. When you research Light Tank II, then in the modeler, you upgrade the Faust to Faust II. At this point, Faust I will show that it can upgrade to Faust II, and Faust II will have -1 for its upgrade. When you research Light Tank III, and upgrade the model to Faust III, then Faust II will show that it can upgrade to Faust III, and Faust III will have -1 for its upgrade. This is because the Faust IV SFT doesn't exist yet. The same should be for your Combat Engineer II.
But that is not what is happening. I agree that -1 is not the problem--what it is pointing to is the problem.
When I first upgrade to Faust I it points to Faust I as the upgrade NOT -1.
Faust I is pointing to Faust II after I research Light Tank X and upgrade to Faust II. Faust II is pointing to Faust I...not -1..
Combat Engineers work but tanks do not. It appears that Fighters also work.
EDIT: that means somewhere in the coding tank is set to point to itself instead of -1...and new versions are set to point to the wrong place.
Not sure why Dive Bombers are even in the model area since the model only uses the dive bomber research to upgrade the Dive Bomber Model...and does not use any of the improvements available in the research area (although ground attack might be nice).