I am using the latest JabRef 5 master build:
on Ubuntu 16.04

Some JavaFX tables get "lazily" updated on Linux: In more detail, e.g. the table in the dialog "Manage external file types" (Options > Preferences > External programs > Manage external file types) is only refreshed, when an updated entry firstly gets scrolled out of the visible table area and then gets scrolled back in, so that the table entry is again visible, then also the values contained in this changed entry are actually shown updated.