Thoracic flexion is absolutely something you should work on improving, so first of all, kudos on the self-analysis.
It is indeed a problem that arises when you lift weights closer to your max, and it's simply caused by the fact that the muscles used to maintain thoracic extension, aren't as strong as the combined strength of the muscles used to actually perform the lift.
Thankfully, there's assistance work to help you get these lower back muscles up to scratch, like back raises, straight-leg deadlifts, and a sleugh of others.
But with that said, it should be noted that the weight at which people do 5 reps per set, is on the high end on the spectrum. If you can do 5x5 at 110kg, then you can probably do 3x8 at 95-100kg, and if you can do that with better form, then that's one way to actually work on your form too.
When it comes to the amounts of weight we pack on when we do sets of 1-5 reps, it's pretty high, and at some point, when you're testing your max, you have to think "just lift the god damn weight". Even though your form breaks, you're still working the muscles involved. It's just that if you make a habit of this, that's when it gets critical.