When I get an implant installed and examine it in the inventory under "Piercings", it says "Quantity: 0" despite it clearly saying "Quantity: 1" in the main inventory before installation. It still shows as being in the slot, but I seem to get no benefit from it, and replacing it just changes what implant there are "0" of in the slot. I can still get the non-existent implant uninstalled, presumably because it registers as something in the slot even if it's "0" of something.