Swap not being used
Increasing swappiness fixed it. The swap was enabled, but for some reason only used when the memory was completely full, and no sooner.
In my case I installed another Ubuntu test instance and formatted the swap for it, so the UUID of the swap device was old in fstab, I just changed it to current and it works.
Don't think so.
First check cat /etc/fstab for your swap partition, then try formatting the swap file system with
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sudo swapoff -a sudo /sbin/mkswap /dev/sd##sudo swapon -a