I have the same phone, an S6 active, and although i do not have the same issue with my phone restarting at full charge and using a wireless charger (my phone works great using the wireless charger) it does not get ANY charge using the micro usb port. Unfortunately, as it may not be so obvious the issue was that the micro usb port for my phone most likely had a short from moisture in the port area. All that can be said about this is that whenever your port indicates any presence of moisture DO NOT plug in the phone charger via micro usb and instead do something to get rid of the moisture, like placing the phone in rice and/or using a cotton swap to gently wipe away any moisture.
To fix the problem of a shorted micro usb port you are most likely going to have to find a way to replace the micro the usb port that lays on the phones daughter board; either replace the micro usb port itself or, probably more practically, the daughter board itself. This is not an easy task and without the right tools or experience I would recommend just converting to a wireless charger like i did but to replace the daughter board you might want to look at Galaxy S6 Charging Daughter Board (T-Mobile).
As for the problems with your phone restarting on you at 100% charge, my guess is that there might be problem with the daughter board or the battery itself.... not quite sure though since I am not an expert. I would recommend the guide i linked though for anyone with a micro usb port that has shorted.