Is it necessary to repeat the prayers that were performed after ghusl from janabah while there was a small plaster stuck to the back, preventing water from reaching it, knowing that its presence was not discovered until after the prayers were completed?

If this patch was applied before the ritual bath (ghusl (definition)) and acts as a barrier, then the ghusl is not complete with its presence, and prayers must be repeated after washing the area of the patch. However, if the time of its application is unknown, and it is possible that it was applied after the ghusl, then you are not required to repeat anything, and the ghusl is valid.