What can be the reason multi-meter shows continuity even though there is isolation?
Looks like there are jumpers, that are not shown as installed, but are installed by default on your back panel: 



Given the comment Peter Smith noted about keeping voltage between I/O common and power common low (&lt; 3V), we might assume there is something like a TVS in there, and the intended purpose of the isolation is just to prevent ground loops, so it's intended to be connected at some defined external point.