With Universal cPlane option (or normal cPlane and using the synchronize command) any modification of the cPlane in the Top view will cause the Perspective to loose its vantage point and become a top view; that is when the perspective projection is set to parallel.
I think this has been like this since ever, but i can’t imagine it being usefull like that. It is probably not reported a lot because tweaking cplanes synchronously while in parallel mode is not a common workflow of choice.
This turns out to be be design - the tool looks for all viewports with parallel projection and changes camera and target positions to be according the new CPlane…
Thanks for checking. I left a comment in the bug tracker. I feel the bug is in the concept itself. The idea of checking only for camera projection is not sufficient to the user experience.
I propose checking the camera vector itself against the axis unit vectors; instead of making a blanket camera projection test.