maybe it would help to identify which FE/E-mount lens was mounted on when the light leak was experienced. or whichever combination of brand of 3rd party adaptor or lens was used that resulted in this happening.
1) the original claim was light leaking around a body cap at ISO 25600 @ 30 seconds shutter speed (other forums)
click here
2) another original claim was when using the Metabones EF-E mkIII adaptor with Canon UWA lenses.
click here
the first link shows the US situation, where the TS mysteriously disappeared shortly after posting the problem. it has also been since revealed that other manufacturers face this issue as well.
the second link shows that it could be the adaptor used that it the main issue.
it could also be an issue that plagues certain batches of cameras, perhaps the initial batch. however this caused a major hoo-hah that had many people panicking. in fact, i myself faced quite a few people who claimed to have light leaks when there was none at all.
suddenly it seemed like some people started seeing unicorns everywhere they go. no doubt i've seen some cameras with this issue, but i believe it's not a large majority who are affected.
i once did testing with someone's A7R with an SEL35F28Z where he claimed there was a light leak when the lens cap was on the lens (ISO 25600 at 30s, wide open). so we tested it together. The A7R with FE 35 F2.8 was placed flat on a table with a strong desk lamp shining down from above hitting the top of the camera and lens. some light would hit it from the sides.
1 - no taping around the lens cap or area around the lens mount.
2 - taped around the lens mount. no tape around the lens cap
3 - taped both lens cap and lens mount
4 - no tape around the lens mount but taped up the lens cap
the only time there was a leak observed was when the lens cap wasn't taped up.
i'm not saying that there are no units that are affected, but there are people who simply are making a big hoo-hah without doing their own proper testing, and scaring the $hit out of other users even though their own units aren't affected.