A couple things, one of which may help, the other may be new information.
- Make sure you're using the latest ESXi embedded (HTML5) host client available from the Fling site (v1.24) as it is known to resolve several issues from v1.21.
- All Gen 7 hardware from HPE is not officially supported for 6.5.
I'll try the update now, however I am not particularly hopeful, the issue is only present on this VM, none of the others have the issue when changing CPU cores. I am pretty stumped as to the cause at this point. I'll swap the SD card adapter over but I doubtful that will do anything.
I'm not too worried about it being supported, generally it isn't an issue. When installing it on my HP ProLaint DL160 G6 with Xeon E5606's, it warned me this was the last versions to support them, but I never got any kind of warming from this host when I was installing ESXi, and it's worked completely fine up until now.
6.5 will work but you get warnings about future CPU compatibility.
DL380 G7 CPU is not compatible for ESXI 6.5.
Gen 7 hardware from HPE is not officially supported for 6.5.
I'd check the content of the VMs configuration file (*.vmx); maybe it contains something strange.