When comparing revision of Lexeme page, diff does not show any differences (while actual changes have been made to the entity), see e.g. http://wikidata-lexeme.wmflabs.org/index.php?title=Lexeme:L5&diff=prev&oldid=12.
There is diff-related code in WikibaseLexeme already. It should be investigated why changes to Lexeme entity do not show up on the diff page.
Once reason for this hsa been tracked down, tickets for fixing the issue should follow.