It's known to affect hba1c. Not sure what you mean by for better of for worse, it becomes less accurate because it gives a lower result.
This doesn't mean your blood glucose has been lower though.
If you time your donations so that there will be 2 to 3 months between last donation and hba1c test it shouldn't make a difference.
I donate blood every few months. It can make your A1C read lower because you are producing new red blood cells that aren’t as glycolated. My endocrinologist knows I donate blood.