Ok, just realized that selecting (highlighting) and replacing the whole field in one go works. Not a good solution as it can sometimes be infuriatingly difficult to get both text selection markers to appear on a tablet (in Chrome at least) after seemingly random combination of tapping and holding and holding, re-tapping and holding, cursing, adjusting tongue angle, tap and hold, hold, hold etc. etc.
It looks like a bug to me after all - I guess they put as much effort into testing/reviewing their website as they do their datasheets...