I have used a hole punch to add 2 holes through the seal as in the photos. This allows water that runs down in the V between the bulb of the seal and the U sect that fits over the body an escape route rather than spill over into the interior. I have punched one at each end of the sill an inch or so after the corner flattens out.
Used a tool like this on its largest setting - https://www.wilko.com/en-uk/wilko-leath ... /p/0343445
The hole punched fully through the seal.
and doing it’s job (ignore the little bit of water on the U section - that was from my finger, not water spilling over into interior)