Could be a million reasons. Forest pack, slate material editor, just a couple potential offenders that comes to my mind first.
Dont use either forest pack or slate material, Im on standard stack material mode (even tough its easy to switch between).
Im really using mostly default common tools (Corona Scatter for vegetation scattering).
Shame there is no override funcion/button. where the interactive render preview can be overriden, to only refresh using a button and not either:
manual camera movement, or just random decisions, not triggered by the users.