Hello anonymous user-5906, apologies for the delayed response here.
Depending on the scenario mentioned above you can try some additional steps.
- As per the troubleshooting guide can you please check if you are using a validated VPN device and operating system version. There might be any incompatibility issue which might be the cause.
- You can set-up diagnostic logging for your VPN gateway and check for TunnelDiagnosticLog and IKEDiagnosticLog to see if anything in particular is causing this issue.
- A packet capture on both sides of the tunnel might help in better debugging the issue. You can refer to this documentation as well.
If above does not help in resolving the issue and given its uniqueness, I would suggest that you create a support request for the same where a support engineer can have a screen share session to pin point the exact issue. If you have a support plan you may file a support ticket. If you do not have a support plan please let me know. Thank you!