Our development team has reviewed this issue, and determined that what you are experiencing is not a product defect - it is expected behavior in Skuid Millau in Lightning. Your iframes are being blocked from loading by Salesforce’s Lightning Lockerservice. This wasn’t happening in Skuid Brooklyn because Lockerservice wasn’t being enforced as strictly (Skuid Brooklyn uses Salesforce’s API 39). Since Skuid Millau is using Salesforce API 41, Lockerservice is now enforced when using Skuid in Lightning.
One possible workaround is to enclose your Skuid page in a Visualforce page, then bring that Visualforce page into your Lightning app, instead of using the Skuid Lightning component. This is a way to put all of the iframes’ URLs inside one domain, but it has limitations because it’s still using iframes, in the end.
Read more here about this change in our release notes, and please see our information about iframes here.
