Hey @JIF, taking a look here. Looks like you are using the return value of a different SQL query for some of the input data. Which returns an object of arrays (the column data for each row returned). I don't believe that this has changed, but will continue investigating.
Using {{ queryName.data.columnName[0] }}would be the recommended reference here. Though interestingly enough in JS stringifying the array (when it only has 1 element) provides the same value. Which you have already discovered.
I'll check in with the team responsible for connections / query result interpolation and let you know if I find that anything was different prior to a couple of weeks ago.