1，怀疑是输入启动瞬间过压造成。建议抓一下Vin波形。
2，原因可能是与输入电容选型有关。图中的C2如果是用的陶瓷电容那么出问题的可能性就比较大了。芯片规格书在“Input Capacitor Selection”中有提到这个类似现象的问题，“A word of caution regarding the use of ceramic capacitors
at the input. A ceramic input capacitor can combine with
stray inductance to form a resonant tank circuit. If power
is applied quickly (for example, by plugging the circuit
into a live power source) this tank can ring, doubling the
input voltage and damaging the LT8631. The solution is to
either clamp the input voltage or dampen the tank circuit
by adding a lossy capacitor in parallel with the ceramic
capacitor.”
3，具体解决方法可以查LT官网上的AN88，再用试验确认。
4，增加的R5可能会有副作用，因为芯片规格书有提到INTVCC不能外接负载，不清楚这大约1mA的电流是否会有不良影响，至少会增加结温。