Beginner: Core strength limiting factor?

I have previous experience training. My all time best deadlift is 315x4 @ bw: 165 lbs. Nothing impressive, but I am proud of that accomplishment. I just got back to training after ~2 years time off.

​​​​​​IN any case, I have always had this issue when deadlifting that is more present at the moment but I remember it even when I was at my strongest. As I fatigue, I have a strange sensation in my abdomen. In some instances, it simply feels like I am straightening my back using my abs at the top of the deadlift. Other times it feels really odd as if my abs slip over something. I wouldn’t describe it as painful, but it is very uncomfortable.

Right now I am really out of shape and I was doing deadlifts and rows in the same workout. In both exercise today, I could feel that sensation of straightening my back. Its this part that is the real limiter to my sets. It makes gauging RPE difficult.

​​​I am leaning towards the hypothesis that I am simply detrained and that this will mostly work itself out over time. However, I have the nagging competing hypothesis that I am executing my deadlift in a manner that unconducive to long-time training (mostly because of how uncomfortable it is) or that I should be doing something additional or more specific to address the issue

Thoughts?

What do you mean slipping over something? Why would your back straightening be an issue in either of these exercises?