I seem to recall this issue coming up previously. I think non-XO snapshots are flagged as templates to that they aren't included in the VM list.
Yes, I just found it here, https://xcp-ng.org/forum/topic/2718/xcp-ng-center-20-03-00-released/ , in the discussion between you, @hvilches , and @olivierlambert
It looks like it's not just XCP-ng Center / XenCenter that create snapshots with the is-a-template parameter set true. The xe CLI does as well when creating a snapshot with the xe vm-snapshot command and some xe commands like xe vm-list show the snapshots with is-a-template set false but not the ones with is-a-template set true. xe snapshot-list shows both types but allows you to select just one or the other with is-a-template=true (or false). xe template-list doesn't seem to allow showing the snapshots by trying to select using is-a-snapshot=true though the parameter is accepted and not flagged as an error.
Odd behavior all around. I only noticed it because I have scripts that parse through the vm-list output and end up catching XO snapshots but not others in the process.
I was originally thinking this might be a bug in XO but maybe it's actually a problem in the xe vm-list command or XAPI in general, i.e. a list of VMs shouldn't list snapshots the way it already doesn't list templates.