The Blueocean GUI doesn't show artifacts if they have the same name as another artifact. For example, the classic Jenkins UI might show the following artifacts:

• tests/1/results.xml
• tests/2/results.xml
• tests/3/results.xml
• stuff.txt

However, Blueocean will only show one of the "results.xml" artifacts:

• tests/3/results.xml
• stuff.txt

The artifacts all exist and can be downloaded via the classic UI. It's quite easy to reproduce by just having some files with the same name and archiving them via a line similar to:

archiveArtifacts artifacts: "tests/**/results.xml", fingerprint: true