Observers do not detect changes in a leave's block distance from nearest stump, when updated by a sapling growing into a tree. First encountered the issue in my Tree farm which was working in 1.15 and 1.14, where the it would detect a tree growing via this method, however in 1.15.1 when going to use the same design I found it no longer worked. I proceeded to test in a creative world and tested the 5 saplings which grow without being in a 2x2 and found that all saplings had the same issue, a screenshot I used for testing will be linked. 

The Dropper and chest were used as a method in order to see if the observer detected any change by placing an item into the chest every time it activated, which in previous versions was when it detected any change to its distance tag.