It was known in advance that HTTP/3 support would eventually be enabled for the load balancers. If you wanted to force support for it or force it to not support it there was an override setting. Mozilla did not override the behaviour meaning that they did not mind HTTP/3 to be advertised as being supported.
so this is my "worked in aws" informed take, but you absolutely do not force default behavior change on anyone unless it is absolutely necessary, and advertising http/3 isn't necessary.
AWS had a similar oopsie with their Load Balancer: introduced a new flag to strip some headers, enabled it by default for all existing public ALBs, and the effect was that all custom headers with an underscore in the name (“x-access_token”) started getting stripped. Took down a few APIs https://forums.aws.amazon.com/thread.jspa?messageID=923182
