I can reproduce this behaviour (AutoCAD2018).
These are what I observed after renaming an layout:
1. The viewport on this layout loses its original size, scale (standard/custom), but its AnnotationScale remains unchanged. In my test, the viewport becomes larger.
2. the viewport would also changed to MSPACE mode if it was saved in PSACE mode prior to layout name changing
3. If there is multiple viewport on the layout, only one viewport changes due to the layout name change.
4. As matter of fact, not only name changing, any change made to the layout in side DB has the same affect to the viewport on that layout (I tried to change Layout.TabSelected property).
Of course, these strange behaviours do not happen when the drawing is opened in AutoCAD and layout name is changed (manually, or programmatically). It looks like if layout object is modified without Editor, something would happen.
It would be interesting to see if the same thing would happen or not when run open a drawing in accoreconsole.exe and modify layout, because open drawing and update in accoreconsole.exe is different from open a drawing as side database, even though in both cases editor is absent. I may try it out if I can manage a bit time later.