Make sure that there is no "purgeOldExtracts" in the replicat prm files -- that would cause the trails to be deleted prematurely.
Otherwise... I can only guess: Is "minKeepHours 2" really doing anything useful? Would it be ok to just use "purgeOldExtracts", with no options? (since, "useCheckPoints" is already the default, and "keepHours 2" is a very small window anyway?) I'm not saying it should be a problem, but it's something to try (as a work-around, if there is a bug... also, you should see if this can be reliably reproduced & open an SR.) If you want to use "minKeep" rules, try re-adding in again (the default) "minKeepFiles 1" (or 2), in addition to the "minkeepHours" rule; maybe the "keepFiles" rule is (erroneously) getting lost. You may also choose to increase the minKeepHours to something like 4 (i.e., greater than half the window from 18:00 to 00:15).
Thanks a lot for your answer.
Finally it seems to be a bug. There's an SR open with it.