No, really at this moment there are not knows bugs with camera clipping and Xrefs, but this is the only Forest feature that hides the items dinamically. Also in the past we had other problems with Xrefs (already solved), because the way that Max handle them, so would be interesting to check it.
Another issue i have noticed is that as the camera passsed some trees they disapear or are unloaded before the go out of view. Would this be another camera clipping issue?
Probably you should adjust the clipping parameters, specially the "Back offset" value. If it is zero, try setting it to 10m. Since these values are dependent of the view, sometimes is neded to use higher values than default.
just to update you this is not related to Xrefing. I imported the camera to the scene containing the trees and the same thing happens.
Ok, in this case, please send us a minimal scene to reproduce the problem here.