With personnel actions on, when you are hiring a worker and the fixed compensation end date is manually set to be later than the position assignment end date, the system let's you to complete the action and assigns fixed compensation with end date which is later than related position assignment end date. Same is happening if you assign the fixed comp manually directly to the worker record should you not do it during the hire process. Fixed compensation is linked to position assignment and for this reason it shouldn't be possible to have an end date later than position assignment end date. Same validation which runs for the start date - there the check works fine.