This problem can be observed when running NoMachine client for macOS connected to a Linux server and also Windows server.

The xev event tool shows how mouse buttons behave correctly if Control key is not pressed down: left button is identified by code 1, right button by code 3.

If Control is held down, both buttons produce events with button identifier 1 (the left button). Any application binding an action to event "Ctrl+Right button" would not work properly being such event identified as "Ctrl+Left button".