After getting the boot ROM built, I was able to fool around with the TI99/4A core tonight. It mostly works great, but I ran into a strange issue in one program, I'd be interested to see if anyone else can reproduce it.
It's the program, "Barrage". If you're not familiar with it, it is sort of a Missile Command inspired program. I have it as a combined bin cartridge image. I took the image right off of the Ultimate GROM multicart on my TI99.
The program loads and initially works fine, but when the projectiles start coming down, rather than coming from random directions, they are all up against the left side of the screen coming down in single-file, in x=0 fashion, rather than having varying X vector components. It looks like the Y component is fine.
I did a full reset of the DE-10, and it happens that way every time, regardless. The missiles that I'm firing up at the incoming projectiles seem to have normal vectors, and the aiming cursor moves normally.