I have deals that can belong to multiple categories. Those categories show up properly in the data editor when chosen from a choice field.
However, in the relation column, only those relations appear that are single relations to one category. On the other hand, the relation field stays empty whenever more than one category is chosen.
Of course, I have checked ‘match multiple’ in the relation settings.
What you need to do is create a Split Text column to turn your Category column into an array, and then use that Split Text column in your Relation column.
The reason that it doesn’t match for multiple categories the way you have it now is because it tries to match the lot as a single string. And of course it doesn’t match anything. Using the split text column as I described above overcomes that - then each category will be considered and matched individually.