Russel, perhaps you could share two routes - one the original and the other the 'adjusted' one. It might help show up that something isn't working as it should. 

Bear in mind that whatever options have been chosen (paved, gravel etc.) will have an impact on all routing decisions that CT will make. 

My understanding is that CT will only change the route before a viapoint if there is some extreme change after that viapoint (dragging the viapoint very far away from the original) or a change in the options. However, I find it good practice if I want to follow a specific route (I rarely do) to add viapoints as I work through it to guarantee that it won't change. (I have the idea that as the algorithm is improved an old route may be 'updated').

As for the 'can't be undone' there is an undo function that works (unless, I think, I've changed the paved/gravel etc. options). 

Changing any part of the route means cycle.travel has to recalculate the route, so if the map data has changed (e.g. a new cycleway) or cycle.travel’s preferences for different types of roads have changed since you first planned it, it’s possible that another part of the route may take a different course.

Usually this isn’t a significant issue because the changes are only very slight, but the gravel routing is a bit of a moving target I’m afraid – I’m not entirely happy with the decisions it makes, and so the routing preferences do change more from month to month than the standard routing. I’m hopeful it will settle down in the near future but for now I’d suggest it’s best regarded as changeable.

Hi, thanks for the prompt feedback. The changes I’m making are within minutes of each other, so I can’t see the underlying mapping changing.  I’m aware of the undo function, but while it will reverse the intended change, it doesn’t seem to undo the unintended, larger changes.
This this original route:  https://cycle.travel/map/journey/517820
the change I’m making is to push the gravel section back onto the V93 between 35-36 miles.  You’ll see from about 15-about 30 miles the route more or less follows the V93 route.
The Updated route is this one:  https://cycle.travel/map/journey/625060
You’ll see the adjustment at new waypoint 2, which is now at 38 miles.  At 35 miles an extra loop has been created but most significant form 15 miles in, the route has now been redirected of the V93 route onto the D12 road and actually the route is quite different right from the start.
This is making me nervous now when planning the long trips I’m lining up. I’m no longer sure that when I sit down of an evening to tweak a particular route - always from the start of the route - that little adjustments later in the route won’t have changed to whole or big chunks of earlier parts of the route, even in the 10/2o minutes that it’s taking me.   Hope the 2 route file links help, cheers.  Russell

Ps I’ve just checked an back along the full route of an earlier stage of this multi-day trip and found that big chunks are different from what I’d planned, presumably because I’d tweaked a small part later on.
https://cycle.travel/map/journey/517549

I am now worried my ability to effectively plan routes.  Cheers, Russ

If you plan a route in standard mode it is unlikely to change significantly.

Gravel mode is not yet good enough and so I am adjusting the “weightings” (i.e. which routes c.t prefers) regularly. That means the routes can change.

If you’re going to need to come back and revise routes after a while, I would suggest not using gravel mode for now.

Hi Richard, chatting with a friend about re-routing, (been using CT in Holland and France recently). If we take a detour from a saved route, re-routing tends to reinvent a brand new route. Is it possible to get CT to offer to reroute back to re join the saved route (which may have been heavily edited in planning) ideally without backtracking? Cheers, Mark