JabRef 4.0.0-dev--snapshot--2017-05-20--master--01e854829
Windows 10 10.0 amd64
Java 1.8.0_131

1. Open a large database (e.g, >6000 entries and several 100 static groups)
2. If you don't wait for a couple of seconds, before you start selecting an entry but instead choose to immediately select one, once the databse is visible (i.e., the graphical user interface of JabRef has appeared), the respective group of the selected entry WILL NOT be indicated by the green number
3. Once that has happened, no matter what entry you select, its group membership will not be indicated by colours in the groups panel
4. To overcome this issue, you have to close JabRef, then reopen it and wait for a couple of seconds, after having opened the database. I guess that some modules (?) are still loaded in the background - if you are too quick in using JabRef, the loading of those modules will be stopped, leading to this behaviour (which was, indeed, quite difficult to replicate, but I think I've now found the pattern)