Overdrive issue

Just bought this Camaro 1989 has a 400 with very freshly rebuild 3 speed with overdrive. When I put it in overdrive it doesn’t shift to lower the rpm but when I put it back in drive it shifts down as it normally would going out of overdrive. It worked fine at first, I was doing 70 at 2500 rpm but then I got off the highway, filled up got back on and I was at 3500 going 55. I filled it up with 85 with about 3 gallons but then filled it up with about 9 gallons of 88 a few miles later. I’m not sure exactly why it changed how it was shifting.