This exact same error is thrown when using the WKND sample site and using iPad Retina in WKND. Galaxy S7 and iPhone 8+ are not erroring out on WKND however.
The JS error originates in the getResponsiveCssClasses function. I notice that editable.config.responsive seems to have different values on my project vs the WKND project, and neither of them have a valid value for iPad Retina.
What configuration might I be missing for this to be breaking?