I fail to reproduce this. Do you have a few example URLs?

https://www.wikidata.org/w/index.php?title=Q5089124&diff=0&oldid=97248895 is one.

(In reply to Lydia Pintscher from comment #2)

https://www.wikidata.org/w/?title=Q5089124&diff=0&oldid=97248895
is one.

Huh, where does the diff=0 come from? Do you typed this? https://www.wikidata.org/w/?title=Q5089124&diff=cur&oldid=97248895 does work as expected.

The core code that checks and parses the diff and oldid parameters is a bit strange. There is no comparison with a constant "cur" in core. Instead it checks for "prev", "next" and numbers and treads everything else (even an empty ...&diff&... with no equal sign) as "cur". Example: https://www.wikidata.org/w/?diff&oldid=104570626

For some reason all this does not work in Wikibase. In my opinion it should be made the same.

diff=0 compares to whatever is the newest revision. It is in email notifications for changes for example.

Change 115310 had a related patch set uploaded by Thiemo Mättig (WMDE):
Wikibase parsing of diff=... parameter was different from core

https://gerrit.wikimedia.org/r/115310

Change 115310 merged by jenkins-bot:
Wikibase parsing of diff=... parameter was different from core

https://gerrit.wikimedia.org/r/115310