For those who may not know, the reason that undoable changes are committed on Save is that in Creo, you can make complex/detailed references from one file to another, and committing the changes is needed for id stability. If we did allow undo after save, we could end up with workflows like these:
There is a drawing D.1 of part P.1.
User 1 retrieves P.1, makes a new feature, saves it P.2, continues their session...
User 2 retrieves D.1 of P.2, sees the new feature, and documents it with detailing, showing dimensions and making a note pointing at an edge made by the new feature. They save D.2 (P is unmodified, and is still checked out to User 1). They leave for the day.
Meanwhile, User 1 undoes the creation of the feature, and makes something different, maybe a cross-section and a different feature. They save P.3 and leave for the day.
The next day, User 2 retrieves the drawing, now D.2 of P.3.... and the ids of the shown dimensions now correspond to a different feature, and may be of the wrong type (say, a radial dimension in the feature where the detailing information is for a linear dimension). Worse, the edge that the note leader was pointing at now resolves to the cross-section... At best, they'll have misleading detailing, and might even crash.
With committing the undoable changes, User 1 can't 'un-create' the feature after save, and needs to delete it instead. This makes the ids for its dimensions/edges know that there used to be something here, but now it is deleted. Then the drawing updates predictably to the changes, removing the now-deleted dimensions and the note leader.
By no coincidence at all, this is also why you should never roll back (delete the latest version on disk or discard changes) to a referenced model without also rolling back a referencing model.
