Well, bug or not it’s been reported as such:
- to GNOME to GitLab - gtksourceview issue #29
- to Ubuntu on Launchpad - #1857191
A recent comment by a GNOME maintainer in the upstream report suggests a fix won’t appear in Ubuntu or in any other distribution any time soon:
These style schemes in question aren’t designed to be interchangeable across any theme. Applications need to choose them wisely based on the current theme and or dark preference.
We might introduce something better in 5.x, but that’s a long way out and will probably come with the additional cost of fixed (rgb, no alpha) backgrounds.