We don’t recommend using an older version of spacedesk. It contains various bugs that are already fixed in the most recent sapcedesk driver version.
If the custom resolution you selected don’t have a * in it, then it does not match the aspect ratio of the device’s screen, sometimes it keeps the aspect ratio (with black borders on the side) sometimes it’s not. This is an inconsistent behavior in spacedesk which is still present.
Could you please try this workaround and check if it works for you:
– on spacedesk viewer settings -> resolutions, enabled both native and custom resolution that don’t have * (e.g. 800×600)
– connect the viewer app
– open the display settings window on your primary machine
– switch the spacedesk display resolution from native to custom (maybe a few times) and check if it will eventually keep the aspect ratio upon switching to custom resolution.