Based on your previous posts in this thread, bear in mind that a straight lumbar spine is already in posterior pelvic tilt.
As for the thoracic spine, people may have pain from hypokyphosis. My understanding is that a t-spine in hypokyphosis may cause the interlocking surfaces of the vertebrae to rub against each other more than normal, but I never looked into it too closely. Given that they’re posterior to the discs though, I can certainly see more pressure being put on them compared to a normally kyphotic t-spine, where the majority of the weight is on the discs which are better designed to take that pressure.
There may also be breathing issues. This is a very informal demonstration, but take a full breath with your t-spine in a neutral position. Then try to extend your t-spine/push your chest out. You’ll notice the pressure inside your lungs rise. Similarly, note the different amounts of air you can inhale with your t-spine in a neutral vs a hyperextended position. You’re essentially flattening out your ribcage, and while hunching over isn’t great for your breathing, hypokyphosis will also cause problems.
Disclaimer: I am not an orthopedic surgeon. I have, however, played around with my fair share of cadavers in anatomy classes and so on.