Bracket tricks in G3

Something new in G3 about brackets that confuses me
Opened this file in G3 and have a different layout for the layers. It seems to work as it did in G2 but why is the layer red as if the interpolation was not working?

The Light master needs a bracket layer, too.

Really, How? I am really confused now

Or you can add a “Enforce Compatibility Check” parameter and uncheck the box. This setting was off by default in Glyphs 2 and is on in Glyphs 3. Mostly because if you like to export as variable font, you need that extra layer (even if the user will never see it.