There should be NO media in the pre-allocation list. I haven't seen a need for this since back in Omniback 1.3
Looking at your description of the problem "... Each day we eject the tapes and introduce the next day pool tapes. the problem happens when there is a holiday day and no one changes the tape..", it appears to me that Data Protector is appending to the tape that was used on Monday in your example. Check your session reports from Monday, and a Holiday Tuesday to see if this is true
If so, I think that the only answer is to change to properties of the Media pool to make the media non-Appendable. You didn't mention if your tape library has a Barcode scanner, but, in the backup specification, in the Properties for one drive enable the Rescan option. if you have Barcode scanning enabled, it should take only a few seconds for the Barcode reader to check what media is available and update the IDB. If you don't have Barcode support enabled, it takes 60-90 seconds per tape, so you probably don't want to do this if you have a library with logs of tape slots
If what you are doing is using the same tapes week after week, the non-Appendable solution probably won't work for you