All arguments which limits the pool by country will generate a custom mirror pool and the mirror pool will be used on subsequent runs.
My personal recommendation is to avoid mirror shopping.
Cherrypick one mirror which provides the best result for your use case and stick with it.
You will create a more stable experience.
The Manjaro package manager will setup a mirrorlist timer which will recreate your mirrorlist at - I think - a weekly interval.
pacman-mirrors uses the Endpoints | GeoJS · REST/JSON/JSONP GeoIP API service
I cannot say what the issue is - I have just tested from my location - it is correctly identified as Denmark.