I do not know a lot about Azure data factory, but looking at the screenshots, it LOOKS to me like you are only pulling 1 column from ParquetSource. When you select that object, it says "Columns 1 total". This makes me think that the JOIN only having 1 column is expected.
Some other ADF experts may have better input, but that is what it looks like to me.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!