I would also like information on this bug.
I have been experiencing the same issue. The primary display becomes unreachable when using more than one display on the Master with the mouse. There are some workarounds I found.
The workaround solution to resolve the issue entirely is to do the following:
On the Master, go to Client Systems > Directory Monitor Setup, then click the radio button "Let Input Director infer which connected monitor is represented by which monitor icon. (Recommended)"
That resolves the bug entirely. However, this is not how Input Director used to work. I'm concerned that Input Director will not be able to adequately infer monitor configuration on a more complicated setup, as in the case with user "V@no". I have no evidence of this suspicion because I have a pretty simple setup.
The other workarounds I mention are related to getting the mouse back on the primary display of the Master after you lose it. If you can use ALT+TAB to select a window on the Master primary display, you can then use ALT+SPACE, press "M" on the keyboard for MOVE, and press the keyboard arrow keys in any direction to take control of the windows by keyboard. After that keyboard arrow key is pressed, you can move the mouse, and it shows up on the Master primary display again. Until it doesn't, and the bug comes back.
Until the bug is resolved, the only way to solve the issue is to enable "Let Input Director infer which connected monitor is represented by which monitor icon. (Recommended)". The bug is a show stopper, and I had to stop using Input Director until I could solve this myself.