The introduction of next-gen projects at the beginning of 2018 changed some Jira Cloud REST API assumptions and core concepts. As a result, some apps experienced unexpected behaviour, few apps stopped working on next-gen projects. We should have communicated this change in behavior ahead of time and moving forward we will do a better job of keeping the developer community up to date on changes that would affect their apps.
App developers and cloud REST API consumers may need to modify their code to support this new model. The blog post explains core concepts behind next-gen projects and ongoing changes to our existing APIs to allow app developers to fully support next-gen projects.