This happens to me too, but usually after 45m - 1hr? I 've bought a brand new PC so I don’t think it’s my end and it only occurs with Forza. What I normally do (which i got from Joey Rocket) is i’ll save periodically so I don’t lose progress. When you wanna save just “place finish line” it will ask you to save your event ( make sure you do v1, v2, v3 etc so you can keep track) and then it will come up with the option to edit the route again. Much safer this way then trying to do it all in one go relying on the Forza gods.
Link to Joey’s guide: My Custom Blueprint Mega Thread - My Tracks, Props, Tips, How To's, Bugs/Work Arounds, Q&A, AMA - #15 by JJOEY_ROCKET