upp is EWMH compliant. Legacy window manager hints are not generally implemented. More entrenched toolkits implement both, and I think upp should too, but I'm afraid EWMH-compliant WM is a requirement at this point. It may be only a page of trivial code to add for this legacy, but someone needs to get around to do it.
You may still have hit a bug, but we needed debug output (stack trace) to know where to look. Your hardware probably has nothing to do with it.