Okay, so I could reproduce and this should be fixed in the next build now (finally!)
The root cause of the problem was actually pretty unusual. When the iPad 3 came out, it had terrible performance, unless you used a canvas width of 2046 instead of the full 2048 (probably due to obscure details of the graphics driver). We duly hacked in a workaround that set the maximum viewport size to 2046 on retina-enabled iPads. Fast forward about 4 years and the iPad Pro is the first retina iPad device with a viewport wider than 2048 pixels! Our hack was still kicking in and bringing the viewport down too narrow. I tested the iPad 3 and it looks like it doesn't need this hack any more, presumably subsequent iOS updates fixed the original problem. So it should all be working as of the next build.
Interesting how workarounds that are essential at the time end up being a problem in the long run. This is why I'm wary of working around OS/browser bugs...