The titanium valves in the Honda 450r engines will tend to stretch when they get hot eventually leading to the face of the valve breaking off of the stem and getting shoved into the head or piston.
Do you mind sharing the size of the stems and the valve spring pressure you were running when this was occurring? Also, what is the head diameter of the 450 valves?
My 07 trx 450 is on stock valves. Stage 3 hot cam. Running kipplewhite spring kit. 10800 rev limit no problem. Ti valves.