I can't add a new env var in the user interface in Environment properties of EB

This issue is likely related to the environment variable size limit in Elastic Beanstalk rather than a bug. Elastic Beanstalk has a limit of 4096 bytes for all environment variables combined (including their names and values).

When you add a new environment variable and it appears to save initially but then disappears after the update completes, it's typically because you've hit this size limitation. The system allows you to add the variable in the UI, but when it tries to persist the change, it fails silently because the total size exceeds the limit.

To resolve this issue, you could try:

1. Reduce the size of your existing environment variable names and values where possible
2. Move some less sensitive configuration directly into your codebase
3. Remove some existing environment variables that may not be critical
4. Use AWS Systems Manager Parameter Store to store larger sets of configuration data, and retrieve these in your application code as needed

Another approach is to use the command line with eb setenv VAR1=value1 VAR2=value2 which might provide more explicit error messages about what's causing the problem.
Sources
Environment variables in beanstalk are removed | AWS re:Post
Environment variables and other software settings - AWS Elastic Beanstalk