I messaged you via PM regarding this, but I want to leave a record here in case others look at this.
@chrisbvt Solution was to inject a delay between loading different iFrames. While this works there is no option to inject a delay when doing a preview in the Tile Builder Storage Device.
So yes, the delay is a factor in identifying the problem. But it is not a solution.
I'm doing this because I'm going to post about it again.