Azure S2S tunnel <> Watchguard onpremise: no traffic passing through after some time of inactivity

Question

Dear, In August 2023 we've built an Azure S2S tunnel between our Azure VNET & Watchguard onprem. We didn't have any issues with this tunnel: it was rock solid for about 6 months. We didn't reconfigure anything in the 6 months because it was just working fine as expected. 2 weeks ago our issues started: the tunnel stays up: no errors are reported in Azure for the connection and our Watchguard appliance does not report any issues, but no traffic is passing through anymore! When we reset the tunnel in Azure, the issue is gone and everything starts working as expected again. When we keep a PC online and start a continous ping between Azure & Onprem: the tunnel stays live! That's how we keep the tunnel at this moment live so we don't have any production interrupts. The issue begins again when we stop the continuous ping and no user is using the tunnel for a long time (during overnight): then in the morning we see that no traffic is passing through anymore. What we already did:

• Checked the DH groups & Encryption types & Integrity settings for phase 1 and phase 2.
• Already built up the tunnel from scratch on Watchguard & Azure: followed all best practices from MS & Watchguard.
• Doublechecked SA lifetime & DPD settings.
• No traffic selectors were defined in Azure in the past: we've defined them previous week for troubleshooting this: but no luck: Ticket is open with Watchguard & Azure support: but no luck for the moment... All we see in Azure is this:

Anyone? Many thanks!