Steps to reproduce:
On liferay 6.0.11 add a wiki entry
Post a comment and vote for it with "finger up"
Post different comment and vote for it with "finger down"
Stop Liferay 6.0.11 and start upgrade to 6.0.12.
Observe that after the upgrade votes are lost.
Please note, this seems to happen not only in wiki but on every other content as well.