My EM guys, Physics types, [from my working days] indicated that the
three dielectric interfaces, adhesive to glass to adhesive, all with
different dielectric coefficients create reflections at the boundaries.

There are four. You couldn't really have an odd number..

Metal-adhesive, adhesive-glass, glass-adhesive, adhesive-metal.

Then there's the tuner box and all that on the feedline to consider.
Not surprising that there's some significant loss in the process.