@Peter_V 

Hi, the IKE endpoint can send a DPD request to the peer to inspect whether the IKE peer is alive, there is an interval between sending DPD requests. If the IKE endpoint receives a response from the peer during this interval, it considers the peer alive. If the IKE endpoint does not receive a response during the interval, it considers the peer dead and deletes the SA.

So is there any sleep mode or power-off of your iPad during connecting to the VPN?

Check if there is a similar issue on other VPN Clients.