Merge train - no "Add to merge train" button while other merge train is running

I’ve enabled merge trains as in this docs
Started a merge train on one merge request, and thus moved the the other merge request, and wanted to add it to the merge train as in the docs
But surprisingly enough I only have a “start a merge train” option, but not an add… I’ve verified the first merge train didn’t finish, so I can’t understand this.

I’d be glad for help with this