There are several things going on, the 18-55mm lens is not a constant aperture lens, so even with all others settings being manual, the metering is affected by the focal length. Even with spot metering being used, you have an area of 18/2.8 vs 55/4.0 for the meter to use. This happens across cameras built by every manufacturer.
You can switch to using constant aperture lenses so that the effect is not as noticeable or use “cinematic” lenses with T-Stops instead of F-Stops to make sure the light transmission is steady regardless of the zoom setting. It is more expensive, but that is the way a lot of people go to solve the issue.
edit:
p.s. Before you ask, I have never gotten a good understandable answer as to why metering should affect the exposure when the shutter speed, f-stop and ISO are all manually set.
Shootinglight got a reaction from mrPeter in Back Button focus (AF ON) in Movie Mode