After the loupe is active in an image, using the arrow keys to select subsequent images makes the loupe disappear (after moving a couple images ahead or behind first image).
It seems related to if the image has to "load," the loupe disappears. Note the little message in the preview says, "loading" the loupe disappears.
See attached video of the problem.
Confirmed. If you click slowly, the loupe remains; if you click quickly, the loupe disappears.
This is still an issue with Bridge BETA 184.108.40.206.