When I do a stasis boost off of a boulder I always instantly lose all momentum and fall straight down to the ground in mid air. Does anyone know what causes this and how to avoid it ???

If that's happening and Link looks weird for a frame or two, it means you needed to reset your ragdoll state.

Shield-jumping bugs out Link's ragdoll state. To reset it, you need to do something that would typically cause Link to enter ragdoll. Not resetting it will cause Link to bug out and fall in place the next time he's supposed to ragdoll from damage (I.E. He won't get momentum).

In runs, the most common way to reset Link's ragdoll is to "flop", which is to board your shield and unequip it after you land on the ground with it. (Example: 23:40 in this video )

It can also be that you are slightly moving when doing the launches. If you move, you will fall down to the ground. That took me a while to figure out, but it was really easy when I learned it. I am talking about old route btw. I can't do tree launches very well.