I did a Battleground today on PlayStation and I got debuffed from another player's Magnum Shot and the time remaining was 49710d 6h 22m 09s. Is this a known bug?
No, I was definitely debuffed. I had the swirling effect around my feet. I could move but I couldn't run. I could Break Free and it would do the animation, but the effect wouldn't go away.
this usually fixes itself when you switch zones. however if you are in BGs, though luck. You will have it until the end of the match. Some status effects get bugged like this, for example burning, poisoned, minor vulnerability, major breach etc. and they would have the effect on you as well.
In cyrodiil you can fix it by going into cheesemonger's hollow (sheogorath volendruung training ground), or to a delve in cyrodiil quickly. However for Bgs, there is no fix. As far as I know these can not be cleansed as well.