Error "Invalid JSON" is shown when JSON in edited in GUI
And it does not disappear. 
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information
The expiration time for new job artifacts in CI/CD pipelines is now 30 days (GitLab default). Previously generated artifacts in already completed jobs will not be affected by the change. The latest artifacts for all jobs in the latest successful pipelines will be kept. More information: https://gitlab.lrz.de/help/user/admin_area/settings/continuous_integration.html#default-artifacts-expiration
And it does not disappear.