This issue has been brought up a few times by end users in our organization. I have been able to further define the problem.
When viewing Notes in CRM 2015, clicking the Note record in the social pane results in the current user being made the Modified By user. However, if the Note is 1,000 characters (with spaces) or greater there is no change if the Note is clicked. Adding or subtracting from the note always results in the Modified By user being adjusted to the current user.
Why is there inconsistency at 1,000 characters? And why are Notes "modified" if clicked on yet not edited?
