capacitor of too low voltage rating can survive overvoltage...but not always?

[Flyback]

We have an LR8 linear regulator downstream of our mains diode bridge (240vac). The capacitor at the bridge output (at the input to the LR8) is just a 100nf, 1812, x7r, 630vdc capacitor. It sometimes blows up at switch on (one in 50 times).
It surely cannot be blowing up because of over voltage above 630v, because the sensitive LR8 would surely have blown up first.
What we suspect is that our board stuffer has accidentally placed a 250V capacitor there, and most often these are surviving in circuit, but every now and again the product is switched on at mains peak, and this 250v part blows up. What do you think?
Do you think this is likely?...ie the board stuffer has accidentally placed a 250v capacitor here, instead of the required 630vdc capacitor?

Surely it’s the only explanation?

Strangely, the capacitor , when blown up, has a hole in it and there’s tarnish on the PCB near it, but when desoldered the capacitor actually still measures 100nf.

LR8
https://www.microchip.com/wwwproducts/en/LR8

 

[alec_t]

when desoldered the capacitor actually still measures 100nf.

Some capacitors have self-healing properties.
Can you remove the cap from a good board and put it in a test rig to establish its breakdown voltage?
Perhaps your board stuffer has been supplied with wrong/fake caps?

 

[schmitt trigger]

I would strongly suggest that you perform a cross section.

You would be looking at "flex crack damage", like the attached image.

 

[ronsimpson]

the board stuffer has accidentally placed a 250v capacitor here, instead of the required 630vdc capacitor?

No way! No one has ever put the wrong part on a board. Probably the fault of an Engineer.