I get this same issue, for me I’d say it is 5% or less. Definitely quite annoying when it happens as I need to re-teleport to the monolith I’m actually trying to run and, as mentioned, lose the rewards from the just completed monolith. I always end up at the Fall of the Outcasts waypoint when this issue occurs.
Same thing happens to me from time to time. Losing rewards is really annoying, especially if they could potentially be nice…
EDIT: I think I have actually found when exactly this bug happens - it’s when you place a portal right by the entrance to the cemetery, I’ll try to keep an eye for it next time to be sure.