Hello,
I can confirm we encountered this issue on several hosts in the past few months.

Config :

• Server is Dell PowerEdge M630
• local storage on SSD + remote SR on ISCSI
• kernel used: kernel-4.19.19-6.0.11.1.xcpng8.1.x86_64
• we use ixgbe (intel-ixgbe-5.5.2-2.xcpng8.1.x86_64) and network cards are Intel 82599 10 Gigabit dual Port (with bonding on XCP NG).
• firmwares up to date (less than 6 months, when we updated to XCP-NG 8.1+).

Additional informations :

• we don't have any VM with memory ballooning
• shutting down VMs does not free memory
• we don't do many operations each day (less than 5 reboots/stop/start).
• size of pools does not matter (bug happened with two hosts and on another pool with 10 hosts).

According to some messages, it seems kernel-alt fixes the issue ... We'll try to switch kernel when we encounter the issue again.