The rubber trim is bonded to the glass leading edge (sealing bead behind it) and its job is to just fill in the pinch weld area gap to the glass. It doesn’t have much of a sealing property or than to direct water away, and visual..
Like the old aluminum/stainless trim was designed to do only as an overlay.
If this was a replacement glass, probably more of a chance for a leaker.
Shy of removing the glass, generally what’s used is black Butyl windshield sealer, it has great stitchion, stays flexible and pliable.
It was used for years back in the day under glass that had moldings against the glass, as well as under and to fill potential gaps for full rubber windshield seals as a minor sealing aid. It doesn’t cure, so it’s not used in modern glass panel installs
Its messy stuff, actually most anything will be to fill in a channel or as a lip seal, so masking off/around is a good practice (more so the painted surfaces), lots of clean rags and a for sure a compatible solvent to help with clean up of residue left on the seal/paint/glass Gloves,, absolutely..
The Butyl with a narrow cut caulking gun tip works well to direct the sealer, applied to body side channel and under the glass edge if possible. I’d absolutely stay away from silicone sealers etc.
When using butyl, you can do the area just around where the leak is, if known,, and if it continues to leak, simply pry up the seal again (would take some work) and continue on around the seal/glass..