New Merge Request contains already merged commits


we are new with using GitLab, so this might be easy and I simply dont understand it.

We have mainly 2 branches “master” and “production”. All changes first go to master and are than merged into production if a new release is needed.

Yesterday we did our first merge request (with squash) via the GitLab UI and everything went smooth. Today I wanted to prepare the next merge request for next week, but all the commits and changes that got merged yesterday are again showing up as changes in the new MR. How can I prevent this from happening? We try to use this feature to have one squash commit containing all the changes for THIS version and not also all the previous changes.

Greetings Fabian