Sorry to hear, you're facing these troubles. Unfortunately, just from the provided video, it's very hard to say what's causing this problem.
Please, be aware that the Camera - Distance Falloff - Density
feature, when rendering a "walk-through" animations, shouldn't be used to prevent any kind of troubles. As mentioned in our online 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)."
If so, disabling it should fix it.
Would you mind to enable & send us the log by setting General - Log Level
in any Forest object in your scene, before sending it to the renderer? The log file is located at:
C:\Users\<user name>\AppData\Local\Autodesk\3dsmax\<max version>\ENU\Network\Max.log
Thanks in advance for any further comments on this issue you can share with us.