If I open the snippet generator using the 'Pipeline Syntax' link found inside a branch of a multibranch project (e.g. my-multibranch-project/master), the credentials drop down is empty:

When I open the snippet generator from the parent (the multibranch folder itself, i.e. my-multibranch-project) - or for any other job - it works as expected.

It does not matter whether I select the git, checkout or withCredentials step. The result is always the same.

It seems like this issue has been around for some time already since I can see this in a rather old LTS version and the latest one. (see environment).

Please let me know in case you need more information.