Is that the whole method that you posted or a snip - could be that some other steps inside are the real culprit(s) ?
Can we get a demo solution which exhibits this problem so that we could run a test here ?
Other than that, I tend to agree with Marcel regarding the boolean test on the relationship itself as this just seems to test for the validity of the relationship and not the related records/data !
Also what platform/version are you on and have you upgraded or otherwise changed the configuration recently ?