Hmmm, you're right that this important breaking change is not in the release notes or breaking changes doc for 7.6.0. I'm terribly sorry about that and am working to get it documented ASAP!
As for fixing your setup to account for this change, please edit both your Filebeat configuration files and add a path.data setting. Make sure to set it's values to unique directory locations. Then you should set filebeat.registry.path to start with ${path.data}/.
Shaunak