Sounds like yet another example of Atlassian making unannounced breaking changes. Someone internally needs to shake things up because this entire platform is unstable.
They did a similar thing recently with changing a pagination cursor from undefined to an empty string: Atlassian Developer Status - Forge Storage delete API was not removing data
Most of these breaking changes are unannounced and unreported. We developers are just forced to waste our time patching these errors (as OP has done here).
When a post-mortem is produced they say “it only affected a small number of apps” and list out remediation steps to ensure it doesn’t happen again. And then two weeks later it happens again.
Also fyi I’d be extremely hesitant to use UI Kit 2 until it’s in GA. Your time will inevitably be wasted again in the near future. This library is used in the “hello world” docs (which I assume is why OP is using it) but it’s still a beta “preview” release which is so unstable that from v9 to v10 they made over a dozen breaking changes mostly consisting of aesthetic naming convention changes: First returns on upgrade to @forge/react version 10