When someone reviews a patch in Gerrit (e.g. CR-2) and then removes themself from the list of reviewers, that CR-2 is removed too. Numerous examples exist (gerrit IDs): 190717, 120811, 175394, 175192, 171562, etc.
While we might not fix this in Gerrit (to be superseded by Phabricator's Differential) it's worth to check that this will not be the case in Differential at least.
(Filed as one outcome of T113378)
I'm not sure this is actually a bug really. At least with the upgrade now it logs this as a comment. Example:
Removed the following votes: Code-Review+1 by Chad <chadh@wikimedia.org>
If you are no more a reviewer there is no point in keeping your vote, afterall you are not going to receive reply / new patchset notifications. If we kept the CR-2 sticky how would the original voter notice it can lift it ?
So either:
A) folks keep their CR-2
B) they abandon the patch saying NO
Looks to me like the notice is good enough.