After some research and looking into the specifics of the circle of protection a bit, it seems like the reflection isn't actually luck based. According to the Circle of Protection entry in the BOIR Wiki, the item:

Surrounds Isaac with a large white halo around him that pulsates.

 

• With each pulse, the halo deals contact damage to enemies equal to Isaac's damage.

 
• Each pulse converts any enemy shots touching the ring into friendly purple homing tears that have Isaac's damage and no other tear modifiers.

This can be seen if you check out the video of the item in use on the wiki page. When an enemy tear enters the ring just as it is getting brighter it gets reflected, otherwise it passes through as normal.

After some research and looking into the specifics of the circle of protection a bit, it seems like the reflection isn't actually luck based. According to the Circle of Protection entry in the BOIR Wiki, the item:

Surrounds Isaac with a large white halo around him that pulsates.

• With each pulse, the halo deals contact damage to enemies equal to Isaac's damage.

• Each pulse converts any enemy shots touching the ring into friendly purple homing tears that have Isaac's damage and no other tear modifiers.

This can be seen if you check out the video of the item in use on the wiki page. When an enemy tear enters the ring just as it is getting brighter it gets reflected, otherwise it passes through as normal.