Just working on my parts libraries and need to use the external text editor a lot, as it’s faster to edit a couple of devices in text file next to each other vs the way KiCAD does it.
Anyhow, when you do that and you use the library browser to check your work, it will never pick up those external changes… if you change something with the part library editor it will load those changes instantly though.
To get the libraries reloaded for external changes you can restart KiCAD completely or just go to:
Part Library Editor > Preferences > Component Libraries
and hit [OK] at the bottom of the window.
This will reload all libraries (inclusive the external changes).