hey thanks for the ping - so after checking with the rest of the team, the answer is that this isn't a bug.
trophy placement takes precedence over legacy global leaderboard rankings. it does not make sense for your best season to actually be lower than a higher result (in your screenshot, the after has higher trophies than the 'before'.)
i know people are disappointed that they lose their highest global ranking in favour of the new trophies but unfortunately it is the way that the leaderboards work - it is always based on your highest trophy ranking.
the trophy inflation through the increased leagues caused these older rankings to become smaller (even though the ranking was higher).
i definitely think that this was something we overlooked in our communication beforehand, and we could have made players aware of this before the change. that's totally on me and i apologise for that.
i know it's not the answer that you wanted to hear but i hope i have explained it in a way that makes it easier to understand. feel free to reply if you need any more info!
drew