if you ask for a halt, and your horse halts but his brain is just holding it's breath until you ask for a forward again, then NO CHANGE has occured. He is halted, but his mind is still focussed on going forward. If you release the reins at that point, you are literally training him to do this again and again. Such that he learns to just lean into and push against the bit.
you should not release the rein/ bit until he actually has some mental change that indicates he has 'thought back to you'. That means he is 'listening' wit his ears and his mind, and has given up that 'go forward' thought , and is waiting for you to say the next suggestion.
I would not release the bit until he has backed up to the point where his ears are pointed backward to me, and he is no longer leaning onto the bit, and if you do release the bit, he doesn't automatically go forward.
I hate to sound mean, but my guess is that you have, inadvertantly, trained this response into him, over many rides.