for instance  if i right click on my vst 3vocal rider the write enble option comes up i choose my parameter(rider fader) and keep plugin open and hit write on the plugin it does not write automation. but when it was a vst 2 plugin it wrote the automation. as for as i can tell it is only with the vst3 plugins not the vst 2 plugs. i did notice it will write automation if all parameters are checked.
 
1 tb
quad hp vs100