Hi I ordered an Adafruit PN532 NFC sheild recently and have had some success with reading NFC tags using a Pi3 and I2C.
However, I’ve noticed that the PN532 requires frequent resetting as my Pi loses all knowledge of it. Could this be a fault with the PN532?
I have no experience with that particular device, but it wouldn’t hurt to post on the Adafruit forum. If you haven’t already. I had one of their SI1145 breakout boards that glitched after running for a while. It did it consistently and they replaced it. Posting there will give you an idea if its an issue that’s happened with that device in the past. My issue was an occasional issue they were aware of. The replacement worked just fine. If they deem it a hardware issue that will make it easier to get a replacement here. ;)
It may also help to post the code your using here. Then those more knowledgeable may spot something that might be amiss.