"sys_metrics doesn't give the real resolution when DPI is active."

I just found this little gem in the manual...
"For retrieving the original desktop resolution, this function (sys_metrics) must be called before switching to a different fullscreen resolution - i.e. in lite-C before any level_load() or wait(), and in C-Script before any call or variable setting that would switch to fullscreen mode."

I think this was responsible for my issue (still to be tested) but perhaps you are calling on sys_metrics after a wait() or level_load() ?