JabRef version 4.3.1 on macOS 10.14.5
Apple's generic app icon is displayed instead of JabRef's icon in macOS Launchpad:

The icon is displayed normally in the dock – depending on the required resolution.
I had to recreate the app.icns file because it is partially corrupted. Could the updated file be added to the pre-build dmg-files?
app.icns.zip
Thank you for reporting this issue. The icon file was indeed corrupted. I replaecd it and it should be avaiable in the latest master version now.
We would like to ask you to use a development build from https://builds.jabref.org/master and report back if it works for you. Please remember to make a backup of your library before trying-out this version.
Commit fixed the corrupted icon. Checked with JabRef_macos_5_0-dev--snapshot--2019-05-27--master--0a4210e2a.dmg.