pmdg appr hold

For some reason when i get vectors to intercept the localizor and when i intercept it and click approach hold it stays locked for most of the final but always turns off when i still have a way to go.. is that normal. Most of the other planes i've flown I've turned auto throttle off pretty far out so i can get my speed right but leave appr hold on till i'm a couple hundred feet out then disengage and land.. but this one turns off about halfway down final and wont re engage.. is that normal its the pmdg 738