It's likely Kim's most recent bomb was too big for the mountain. Maybe the tunnel wasn't deep enough.
From the Wikipedia on the test site:
2017 tunnel collapse[edit]
On 1 November 2017, Japanese TV station TV Asahi reported that according to unconfirmed reports, several underground tunnels collapsed at the test site on 10 October 2017.[14] An initial collapse was said to have killed 100 workers, with another 100 rescuers killed in a second collapse.[15][16] A 17 October 2017 study published by the US-Korea Institute at Johns Hopkins University suggested the most recent test had caused "substantial damage to the existing tunnel network under Mount Mantap".[17][18] On 30 October 2017, in testimony before the South Korean parliament, the director of South Korea's Meteorological Administration warned that "further tests at Punggye-ri could cause the mountain to collapse and release radioactivity into the environment."[18] Likewise, Chinese scientists warned that if the mountain collapsed, nuclear fallout could spread across "an entire hemisphere."[18]
From the Wikipedia on the most recent test (on 3 September):
On 13 September, an analysis of before and after synthetic-aperture radar satellite imagery of the test site was published suggesting the test occurred under 900 metres (3,000 ft) of rock and the yield "could have been in excess of 300 kilotons".[22]
Thanks...Yes, I should have been more specific.
I was referring to the effects of radiation on the people in the large areas surrounding the ongoing testing.
If indeed their are birth defects it probably has been been occurring for some time.
Of course I would guess there are not a lot of Norks who are successful in defecting.