Pay attention to whether the EOFB appears more obvious at low power or at high power. If this is the case, it's appearance or severity might be affected by exit pupil, rather than the act of zooming itself.
When I first acquired a Leica Zoom, I recall comparing it with my set of XW's. On nights of poor transparency, the XW's would show slight EOFB, while it was much more apparent in the Leica. The transparency had to be excellent for me not to see any EOFB in the Leica. That doesn't happen very often around here.
this EOFB observation sounds to me like marginal rays of scattered light entering the eyepiece, and generating the edge of field brightening.
The spotting eyepieces don't have the insert tube properly protected (with the buffles, rills, etc.) like most of the astronomy eyepieces, and especially not when you are not using the original adapter bayonet-to-1.25" or -2" of the manufacturer.
The baffle(s) to cut the marginal scatterd rays are is in the spotting scope.
That's the reason why I have added at least one baffle in the diagonal mirrorrs and prism houses for my 6" F/5 achro
What also helps on the nights of lower transparency are the oversized lens hoods. I have them on my 6" F/5 achro, on the Leica 82mm APO Televid, and on my astronomy binoculars.
In fact, I mount these lens hoods always.