Thank you for asking this question in the support community. Those messages typically mean traffic is not communicating in both directions (which could be due to a poor connection in this case). However, if it happens consistently when the connections are stable, I would be happy to review the configurations of the VPN peers and evaluate the output from the debug crypto ike with you.
Please, do not hesitate to reply to this post with any additional information or questions. I will be happy to help in any way I can.
It may be confirmation bias, but I think this is the right answer. Just needed someone more familiar with IKE/crypto errors to confirm.
Also thanks for the Debug Crypto Ike tip!