I had the same issue last year when I was using LGP and OBS quite often. Never found a solution other than setting up everything before I started streaming. Also when LGP is added as a source, same thing happened when I tried to adjust something with my Logitech G920 webcam. Without LGP on the scene I can set up the camera's properties no problem while streaming.
Now that I think about it, I might have found a workaround but I can't vouch for it - you'll have too try it yourself. Try disabling LGP pefore opening properties (untick on the sources list), enter the properties and re-enagle it on the scene. This might have worked for me at some point but I really can't remember now.