This creates a serious footgun in Ubuntu netplan, leaving a server totally unbootable, but simultaneously not triggering "netplan try" as any sort of parsing problem:
Getting in the habit of not doing so will lead to schema violations, like the Netplan problem you linked, which can crash the program trying to read your config. If it bails out at an unfortunate time, like most networking tools seem to do, you'll need to use a recovery boot image or serial console to fix your config.
% echo '{"address": "2::0"}' | yq -P -
address: 2::0
String with ": " inside have to be quoted, that's correct, however you won't have space in your IPv6 address unless I miss something.
Been there, done that. A good config file or linter should’ve complained and not allowed me to commit such misstake.