I did a quick search on this bug, but either it hasn’t been addressed or my search-skills are far behind. If I did make a duplicate, feel free to remove/merge.
So the Signet of Malice signet has been buffed back in the March patch if I remember correctly. The healing should be ~33% more effective.
The 33% has indeed been added although the tooltip did not receive the update.
Video link
Server: Whiteside Ridge
Race: Asura
Profession: Thief
Level: 80 (bug is also present when downscaled)
Should be an easy to fix bug.