We've updated Bitbucket branch source plugin from version 936.1.1 to version 937.2.3 and after that plugin webhooks stopped triggering builds in MultiBranch/Org Folder projects.  Bitbucket branch source plugin also caused update other plugins on which it's depends (scm-api, branch-api, git etc.) and we see that SCM source id was changed from UUID to Int in branch-api plugin, so maybe that causing issue?

We're using Post Webhooks plugin in Bitbucket, so configured CasC to use pluginWebhook, not Native Webhooks, since we're not ready to migrate to Native webhooks yet.

In bitbucket-branch-source plugin logs we see that webhook is accepted and processor for webhook found. But after that Branch Event not fired and no build triggered. We also updated webhook url accordingly (added server_url parameter to url).