why was the git log graph of the merged branch from gitlab inverted?

There is something fishy with the two successive merge commits merge branch dev into 668-* and merge branch 668-* into dev.

Try running :

# you don't need the full history : '-5' will limit the log view to 5 commits
git log --oneline --graph -5 dev

You should see a sequence of commits and merge, where at some point dev was fast forwarded to 668-*.