Thanks Mootari and Brian for your suggestion,
I made a few adjustment of notebook comparison, and I found the inaccessible comparison link happens under the following condition.
Notebook1 is public (latest version 156).
Create a fork Notebook2 (version 158) from version 156 of Notebook 1 and publish
Click on “compare with fork” to compare “Notebook2 to Notebook1”
This diff page shows page not found when opened in another browser, although version 156 and 158 are both public.
Currently I solved this issue by the other way around, i.e., compare “Notebook1 to Notebook2”.
In the Notebook2 v158 from step2,
Go to View History, select the very first version (v156) when it was forked out from Notebook1
Fork this version (as if it is same as Notebook1) to Notebook3 (v158)
Click “compare with fork” which compares “Notebook3 (which is Notebook1) to Notebook2”
This link is able to share
Is this same as what your explained?
Another scenario which shows error page is when comparing within a public notebook, i.e., comparing history to its latest, in above Notebook3, when clicking “view history” and select version 156, then “compare to latest”
The above link is also not shareable although both 156 and 160 are public.
It seems currently the only working solution to share a diff page is to publish, then go to history, fork out, publish, edit, and then compare with fork.
Please correct me if i’m wrong.