I'd also like to throw in that i'm experiencing similar issues on iOS devices since sandboxing went live.
In my report I have a few "Card with State" visuals on the initially active page which fail to render, though I see blank grey rectangles rather than the Power BI logo. If I navigate to another page and come back, they do then render, and continue to respond to changes on the page.
The "Chiclet Slicer" is also failing to render initially in exactly the same manner on that page, as well as one of my own visuals. Unfortunately mine doesn't render even when I return to the page, but the chiclets and cards do. I can't discount that my faulty visual is breaking the others, but that would seem a rather counter-intuitive to now happen given what you've tried to achieve with sandboxing. Finally, I have another visual within the same report that only appears on initially hidden pages, and that is also failing to render entirely on iOS. Other visuals on those pages render without issue.
Are there any suggestions as to how to debug a custom visual that works successfully on the web but not on iOS? Preferably something that doesn't involve having to hand over a visual to MS to fix, because that's not going to be sustainable.
On this topic, did I miss an eventual announcement that sandboxing was live, or were we expected to keep watching until something broke? The developer blog was the perfect place for all of this to be communicated, but you've for some reason abandoned it and merged it with the main blog which you seem to want to keep high level and limited to product promotion and feature announcements. A little additional communication beyond a well hidden single thread on the community forum would have helped. Only so much however... as has been pointed out, we couldn't test and debug on mobile platforms, and that's clearly showing now.