Pretty common near me, especially on public land. Example, a lot of the state WMA's were added onto piecemeal, so the first couple hundred acres, then another 50, then 100 here, etc were added later to build onto the existing piece. End of the day a single state WMA might have originally been 4 or 5 or more different properties 50 or 75 years ago--that's actually quite common. In most cases they took fencing, etc down, but not all--any interior fence between pastures that grew up and didnt get a crew to specifically take it down, is still there across the middle of that parcel. Also, it seems they used to put perimeter fencing up around the state WMA's in many areas, I assume to keep neighboring livestock out--but if the WMA has been added onto, that fence that used to be the perimeter often wasn't taken down, and now runs right through an area that is clearly not even near the boundary of the new state land parcel. This used to throw me for a loop too before I started using a mapping app with parcel boundaries. It seems everyone is using them now, so I'm not sure it's keeping anyone out of those areas at this point.
FWIW onx (as well as other similar apps I've used, its not just onx) property boundaries are often a few feet off from the satellite image for me, and perhaps due to frequently poor cell service(?) the gps location on mine is often very imprecise and constantly bouncing around, so I feel like the mapping app can only be accurate to within a handful of yards at best--usually that's plenty accurate, but in a few cases where I want to see if a specific tree or feature is on public or not I dont think it's accurate enough. I've walked my own property boundary with onx in hand and tried to use it to find some of the old intermediate survey marks, and it's close but definitely off by 20 or 30 feet.