Sorry to hear you're facing these troubles. Might be, you are using the Camera - Distance Falloff feature?
When rendering a "walk-through" animations, this feature shouldn't be used to prevent any kind of troubles. As mentioned in our on-line reference guide
"Be careful if you use falloffs with camera motion, because items may appear or disappear suddenly (the distance to camera changes when moving)."
Also, I would suggest you switch to the Custom Edit mode to see if there is any difference.
Optionally, you can do a quick test using Forest Tools and export all scattered geometry to instances. You can find it under Utilities - More ... - Forest Tools.
For more information about it, check our on-line reference guide
. This way you can check that the problem happens during Forest object conversion or not.