Thank you @DaneNagy, glad my discovery helped you.
Since it is effecting more than our system it certainly a bug and I hope it can be fixed.
Question: exactly which version are you running?
If you know how/where the error is: please post it here? or the fix?
Sounds quite interesting that a % char in the NAME or DESCR changes the way it works ... can it be that you re-opened the rule and saved again? or that it took some time to re-run the rule via the indexers?