Debian doesn't usually use netplan? It's written by Canonical for Ubuntu.
This is now part of the cloud images for Debian.
And then OVH makes you hand-configure your IPv6 address, with netmask and IPv6 gateway - no DHCP6 for their VPS servers. Then they put the footgun in your hand by not mentioning the double-quotes requirements which YAML has for IPv6-with-netmask.
Brought to you by the creators of Snap. Yeah that's probably the real issue. Don't touch anything by Canonical.
Are you sure? Debian from what I understand uses standard /etc/network/interfaces