I think the problem is that the position tuple is loaded as a string from the conditions file, so cannot be interpreted as a number. If you instead have separate columns for x and y, and just add the relevant numbers, you can get around this issue. E.g., you have two cols in your conditions file, posX and posY, and in your shape stim you set the position to update on every repeat, with the following entry: [posX, posY].
Just for info, JavaScript will not read a tuple as a list in the same way Python does. You need to use the array format (square brackets).