I THINK I know what you are experiencing. When I use my special prescription, and just relax and keep both eyes open, my sight alignment and picture goes haywire because my brain is focusing right eye on the front sight, and left eye on the target--it doesn't know where to settle, and with the two different prescriptions for each eye, they are ping ponging back and forth trying to pick one or the other. So, at the range, just bench shooting, I wear the glasses for about 5-10 minutes before I shoot, load some magazines, just go about my business so I forget I'm wearing a funky prescription set. When I shoot, I admit I squint just a bit, not hard, to get my brain to think right eye. the left eye is open just enough to give me a picture of the bullseye. I shoot fine.
In run and gun (USPSA), my brain doesn't get fooled at all. I am wearing these glasses the minute I leave the car, and by the time I'm shooting, it's almost like I have perfect 20/20 again. It has finally started working so well I'll have a special set of prescription shooting glasses made (the set I have are "fashion" framed).
YMMV, of course...