Exported images darker than originals

[rich2005]

I suppose it will depend on the processing done. You already checked between bit depth and perpetual(sRGB) and linear modes. Same menu is a toggle for Color Management on/off and if you look in Edit -> Preferences, Color Management is much wider than with Gimp 2.8 Can you remember how Gimp 2.8 was set up?

Another possibility is Gimp 2.10 layer modes, for 2.8 equivalence change to legacy mode.