In a other topic[1] @stgraber said “snap doesn’t actually use /etc/subuid and /etc/subgid”. But nowhere in the documentation that is made clear.
Looking at the source code (idmapset_linux.go) I would swear it does read these /etc files. But it seems snap is doing weird stuff to Linux programs.
[1] Snap lxd: how to restart lxd without restarting containers?