I believe I have solved the problem... to an extent.
To start, I wouldn't call this officially solved by the standard definition since it was kind of a hack.
The first thing I did was I completely uninstalled the IXEG 737, the MU-2B, and the Saab 340, cleaned the /Aircraft/X-Aviation directory, and removed the Gizmo plugin from /Resources/plugins.
The second thing I did was setup to new, and isolated X-Plane 11.26. One for a clean install of the Saab 340, and the other for the IXEG 737.
The third thing I did after installing each plane in their sandbox was compare the installed Gizmo plugin files in /Resources/plugins, and found that the Saab had a couple files with older dates than the same files with the IXEG sandbox.
The fourth thing I did was try trading Gizmo plugins between the two sandboxes. The IXEG took it fine, but the Saab 340 was still suffering the same issues. What ended up working was a merge of the two Gizmo directories, and using the newest version of each file.
Again, I don't know if I would call this an actual fix since I don't know if it was just a fluke, or just some dumb luck, but hey!