Looks like it works when I start the build manually. I’ll poke Ben Cooksley if there’s anything logged in KDE GitLab.
The events correctly arrive at buildbot but it fails to assign proper ID to it and fails to start the build. I’ll add it to my todo, in the meantime you can start it on your own by logging in at https://flathub.org/builds/#/ with your GitHub account, pressing Start build in the top right corner and entering org.kde.Sdk/5.15 into App ID field.