Holy cow, I'm having the exact same glitch, too. It sounds to me like you were trying to do the armed escort mission that starts in Mansa Market? That is the armed escort mission that becomes available after you liberate the first bell tower in Northern Kyrat.
The same thing happened. I rode an ATV to the waypoint, and when I dismounted, the game said I was too far from the waypoint, and the mission failed. Now every time I load the game, it is stuck in an infinite loop of mission failure.
It sounds to me like the mission is glitched, at least on xbox one. That makes me hopeful that they will actually fix it.
Can you do me a favor and verify that the mission in question is the armed escort out of mansa market? Even if you are stuck in the infinite loop, you can still open up your map and get an idea of what side mission caused the error.
If it matches up, I'll start a new thread to try and direct attention to the issue.
Thanks!
ALSO, PLEASE SUBMIT A TECH SUPPORT TICKET IF YOU HAVE NOT DONE SO ALREADY!
https://support.ubi.com/en-US/AskQuestion