When you change the flow rate in a gradient while keeping everything else constant, you are really changing a lot more than it appears on first glance.
When you decrease the flow rate in this case, you are creating a smaller gradient volume, and you are making the gradient steeper. This may or may not change the selectivity of the separation, but it definitely decreases the true retention factor at the column outlet. This makes all your peaks narrower (in volume units), which in turn will give you a better response factor (higher peaks). However, the resolution may get worse, since the distance between the peaks may decrease more than the peak width (both in volume units).
Overall, the expectation is that you will loose resolution, but this may depend on the details of the elution pattern of your analytes. You have seen this: you lost resolution at the beginning of the chromatogram and gained some at the end of the chromatogram.
In principle, you should be able to observe the same pattern as you have observed at 0.8 mL/min by running the chromatogram at 1 mL/min with a gradient that has only 80% of the run time of the original gradient. (There are additional complications due to gradient delay volume that can create additional effects, but let us not worry about this at the moment.)
