When a potion effect arrow is picked up after being shot off, you pick up a full stack of that arrow.
Steps to recreate:
1. Shoot a potion effect arrow at the ground (or miss with it)
2. Walk over to it and pick it up off of the ground.
3. There will be a stack of that arrow in your inventory.
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-82859. You can find or add further information on that report.
Please search before reporting any bugs, as it's likely that one exists already.