I personally don’t suggest to change the data attribute assigned by Bricks, unless we can confirm Bricks’s JS wouldnt use that, but pretty sure your screenshot error causing by that.
Check the instance Id like cmstew suggest.
And change like this
Alternatively, just use another way to retrieve the instance Id like this.
const mainCarouselId = document.getElementById('primary').dataset.scriptId ,
thumbnailCarouselId = document.getElementById('secondary').dataset.scriptId,
main = bricksData.splideInstances[mainCarouselId],
thumbnail = bricksData.splideInstances[thumbnailCarouselId]
@cmstew I think the current way of remaining the bricks’ Id as instance Id is better, can imagine if people assigned same CSS Id for different sliders will cause more problem later 