If it is as usual, some internet western elk hunting master will be along shortly to inform you I am incorrect. Just remember that ignorance is bliss as they say.
I'm a landowner with trespassing problems (like all landowners). I also have used OnX and Gaia. You are having two issues here. Mapping software and real life.
The mapping on Onx, Gaia, etc is only as good as the county's tax maps. The county is not motivated to make the maps accurate, so in many cases they can be off by up to 100-200 feet. But this doesn't really matter.
What really matters is that the landowner is absolutely certain where their boundaries are and that POSTED signs are visible. Then the law can be enforced. Yes, I know that the signs shouldn't be necessary but the judge will be a lot less likely to prosecute if the landowner hasn't posted his property.