Linux has two different kinds of “used” memory. One is memory allocated for/by running processes that cannot be reclaimed or reallocated to another process. This memory is unavailable. The other kind is memory used for caching (ZFS, write-back cache, etc) that can be reclaimed and allocated for other things as needed. Memory that is not allocated in any way is free. Memory that is either free or allocated to cache is available.

It looks like htop only shows unavailable memory as “used”, while proxmox shows the sum of unavailable and cached memory. Proxmox “uses” 11 GB, but it’s not running out of memory because most of it is “available”.